NBN battery backup: what keeps working in a blackout?

When the power goes out, the NBN goes with it, and for many people the first they hear of it is a home phone that will not dial out, or an NBN box beeping in the hallway in the middle of the night. The old copper phone line drew its power from the phone exchange; the NBN runs on the power in your house. If you are not sure how your phone is wired to the NBN, get an NBN technician to check how your phone is connected before the next storm, not during it.

Does the NBN work in a blackout?

No. NBN says any equipment connected through the NBN will not work during a power outage (opens in a new tab), and that covers the internet and a home phone that runs through the modem. The only partial exception is a fibre to the premises home with a battery backup, where the NBN box keeps going for a few hours.

That is why NBN’s own advice is to keep a charged mobile phone nearby, and it is how you reach 000 in a blackout. The ACMA says (opens in a new tab) most NBN services will not work in a power outage, and that a 000 call from a mobile is carried on any available mobile network, not only your own carrier’s. In a blackout across a whole area, the equipment in the street or the building can lose power as well, so a battery or a backup power supply in your home will not bring the service back until the network has power again.

What needs power on each type of NBN

Every type of NBN has at least one box in your home that plugs into a power point, and some have equipment in the street that needs power as well. Here is what has to stay on for the internet to work.

What needs power for the NBN to work, by connection type

Connection	In your home	Outside your home	Does backup power at home help?
Fibre to the premises (FTTP)	The NBN box on the wall and the modem	NBN's fibre network	Yes. The battery backup, where you have one, keeps the NBN box going for up to about five hours. The modem needs its own backup.
HFC	The NBN box and the modem	NBN's equipment in the street	Only while the street equipment still has power
Fibre to the curb (FTTC)	The NBN box, which also powers NBN's unit in the pit, and the modem	NBN's equipment back from the pit	Only while the network still has power, and the NBN box and the modem both need backup
Fibre to the node (FTTN)	The modem	The node cabinet in the street	Only while the node still has power
Fibre to the building (FTTB)	The modem	NBN's equipment in the building's comms room	Only while the building's NBN equipment still has power
Fixed wireless	The NBN box inside and the modem	The tower	Only while the tower still has power

Pick a home

Fibre, with a battery backup Fibre, with no battery HFC, curb, node or building Ready for a blackout

12345678910

1. 1NBN box
2. 2Battery backup
3. 3Modem
4. 4UPS
5. 5Cordless phone
6. 6Corded phone
7. 7Security alarm
8. 8Medical alarm
9. 9Mobile phone
10. 10Street equipment

Fibre, with a battery backup

Still working

• The NBN box, for up to about five hours
• A corded phone, where the provider has set it up that way
• A charged mobile

Off

• The modem and the WiFi
• A cordless phone's base
• Any alarm that dials through the modem

Fibre, with no battery

Still working

• A charged mobile
• An alarm that dials over the mobile network with its own battery

Off

• Everything on the NBN

HFC, curb, node or building

Still working

• A charged mobile
• A mobile-network alarm with its own battery

Off

• Everything on the NBN, and backup power at home only helps while the street equipment has power

Ready for a blackout

Still working

• A charged mobile and power bank
• A medical alarm and a security dialler on the mobile network with their own batteries
• On a short outage, the NBN box and modem on a UPS while the network has power

Off

• The rest of the house

Fibre to the curb is the odd one out: the fibre to the curb NBN box powers NBN’s unit in the pit out the front as well as your own service, so when it loses power the street equipment for your line does too. And if you do not know which type you have, which NBN connection you have shows how to tell from what is on the wall.

NBN battery backup: who has one, and what happens now

Only fibre to the premises homes ever had an NBN battery backup. It is a separate power supply box next to the NBN box with a 12 volt battery inside, and it keeps the NBN box going in a power cut. NBN stopped offering it in June 2024. If you need one now, NBN’s battery backup page (opens in a new tab) says to ask your phone and internet provider, which may be able to arrange a battery backup unit.

What the battery does, from NBN’s own pages:

• It keeps the NBN box going for up to approximately five hours. About three hours come from its emergency reserve, and about one and a half more after you press the Battery Emergency button.
• It powers the NBN box, not the rest of the house. NBN says each mains-powered device needs its own backup. The modem, a cordless phone’s base and anything else on a power point still go off.
• A phone needs to work without mains power. NBN says that without the battery backup and a correctly connected corded phone, the phone will not work in a blackout.

The battery was never automatic for everyone. As far back as 2014 the government’s own impact statement recorded that including one was up to each provider. If yours has gone flat or been taken out, that is not a fault: the NBN box runs perfectly well on mains power without a battery. The battery is only there for a power cut.

Your NBN box is beeping: what to do

A beep every 15 minutes and a red light next to Replace Battery mean the backup battery needs replacing. NBN says four beeps a minute means less than half its charge is left. Here is the order we suggest on the phone.

1. 1

   Check whether the power is actually on

   In a blackout, the box is doing its job: it is running on the battery. If the power is on and the box beeps every 15 minutes with the Replace Battery light red, the battery needs replacing.

2. 2

   Ask your provider first

   Some providers arrange the replacement. Ask yours before you buy a battery yourself.

3. 3

   Buy the right battery

   It is a 12 volt, 7.0 to 7.2 amp hour sealed lead acid battery with F2 terminals. NBN lists specialist battery retailers, and some will fit it for a fee.

4. 4

   Swap it and recycle the old one

   It is a straightforward swap. If the old battery is swollen, hot or smells, don't touch it and keep away from it: ring your provider, and if it looks really bad, switch the unit off at the power point if it is safe to. NBN says a used sealed lead acid battery is hazardous waste: take it to a recycling facility, never the household bin.

5. 5

   Or stop the service

   If you no longer want battery backup, tell your provider so the reminders stop. If nobody else at the address uses it, the battery can come out and be recycled.

If the lights on the NBN box are off after a new battery goes in, check the power before anything else. Push every lead in firmly, and try an extension lead from a power point you know works, because a loose plug or a dead garage circuit looks exactly like a dead box. If it is still dark, ring your provider, which books NBN. What the NBN box lights mean covers the other lights on the fibre box.

If your provider does not supply the replacement, this is the battery type the unit takes. Check the label on the old one matches before you buy.

We earn a small commission if you buy through our Amazon links, at no extra cost to you. As an Amazon Associate we earn from qualifying purchases.

Gear mentioned here, on Amazon Australia

Product	What it is for	Link
FEDHQ 12V 7.2Ah, F2	The NBN backup unit size	View on Amazon for the FEDHQ 12V 7.2Ah, F2 (opens in a new tab)

Will my home phone work in a blackout?

On most NBN homes, no. The home phone now runs through the modem, so when the modem loses power, the phone goes with it. That is the biggest change from the old copper line, which drew its own power from the exchange and kept working in a blackout.

There are three ways to keep a phone in a power cut:

1. A charged mobile. NBN’s own advice, and the one that works for most people. Keep it charged, and keep a power bank charged too if storms are forecast.
2. Fibre to the premises with a battery backup and a corded phone, where your provider has set it up that way. A cordless phone’s base needs mains power, so it does not count.
3. A separate mobile phone unit with its own battery. A small box with a SIM inside that a normal phone plugs into. It does not depend on the NBN, and it keeps going on its own battery while the mobile network is up.

Whichever you have, use a mobile for 000 in a blackout.

If the phone stops working at a wall socket after a blackout but works when plugged in at the modem, the problem is the wiring between them, not the NBN. Phone line repair traces that, and a new phone point is the fix when the old socket is in the wrong place for how the house is wired now.

A charged power bank keeps the mobile going through a long blackout.

Gear mentioned here, on Amazon Australia

Product	What it is for	Link
Anker Zolo 20,000mAh	Keeps the phone charged	View on Amazon for the Anker Zolo 20,000mAh (opens in a new tab)

Medical alarms and the NBN

If someone in your home relies on a medical alarm, speak to the alarm company and your phone and internet provider now, not after a blackout. An alarm that dials out through the NBN stops when the power or the NBN goes down. NBN says a newer alarm with its own backup battery that connects over the mobile network can keep working through a power or NBN outage, and that is where the alarm companies have been moving. An alarm moved onto the mobile network years ago may have used 3G, and the 3G networks have closed: the government’s 3G switch-off advice named some personal medical alarms among the devices that needed upgrading.

NBN’s Medical Alarm Register closed on 30 June 2023. It was set up in 2014 to help people move their alarms off the old copper lines, and it is not an ongoing service, so there is nothing to register with NBN now. NBN’s page on medical alarms (opens in a new tab) sets out what to ask. If someone in the home has a life-threatening medical condition, ask your provider about Priority Assistance. NBN says Telstra is required to offer it, other providers may offer something similar, and on fibre to the premises a battery backup is needed for it.

Back-to-base alarms, fire alarms and lift phones

A security alarm that reports to a monitoring company through the NBN modem stops reporting when the power goes. Our technician has made the same point on the phone for years: if the alarm depends on the modem, cutting the power to the house silences it. The answer is a 4G dialler fitted to the alarm panel, with its own battery, so it keeps reporting over the mobile network when the power or the NBN is down.

NBN’s advice is to check with your equipment provider whether monitored fire alarms and lift emergency phones will work over the NBN in an outage. Lift emergency phones should not depend on the NBN at all; they go on a 4G lift phone unit with battery backup. We fit the same kind of 4G unit with battery backup for a home phone, an alarm, or anything else that used the old phone line, not only lifts.

If someone at home needs a phone or an alarm that works in a blackout, say so when you ring us, before anything is booked. The right answer depends on which service you are on and what the phone or alarm is for.

Can a UPS keep the NBN running?

A UPS, an uninterruptible power supply, is a battery box that your equipment plugs into, and it can keep the NBN going through a short blackout at your house, but only while the network outside still has power. NBN says another power source cannot restore your service if the network is also without power, and in a big storm it often is.

If you do use one:

• Put everything in the chain on it. On fibre to the premises, HFC and fibre to the curb, that is the NBN box and the modem. On fibre to the node and the building, it is the modem.
• Remember what it is for. A UPS buys time through short interruptions. It does not replace a charged mobile, and it is not the way to keep an alarm reporting: a dialler with its own battery is.
• Size it for the boxes, not the house. Plug in only the NBN box and the modem, so the battery goes to the internet and nothing else.

These are the UPS sizes that suit a modem and an NBN box. The small 12 volt one suits a modem with a plug pack; the desktop ones have Australian power points.

Gear mentioned here, on Amazon Australia

Product	What it is for	Link
APC Back-UPS Connect 12V	For modems on a 12 V plug pack	View on Amazon for the APC Back-UPS Connect 12V (opens in a new tab)
APC Back-UPS BX750MI-AZ	Desktop UPS, AU power points	View on Amazon for the APC Back-UPS BX750MI-AZ (opens in a new tab)
CyberPower UT650EG	Compact UPS, AU power points	View on Amazon for the CyberPower UT650EG (opens in a new tab)

When to call a technician, and when you don’t need one

You do not need us to replace an NBN battery, to stop a beeping box, or to buy a UPS: those are yours to do, with your provider’s help on the battery. A box with no lights after a power cut, once you have checked the power, is your provider’s to book in. And a medical alarm is a conversation with the alarm company first.

Call us when the problem is in the house: a home phone that stopped working after the NBN went in or after a blackout, phone sockets that need moving to where the modem now sits, or help setting up the phone and the modem so the right things are plugged into the right ports. If the provider says your line fault is your wiring, an independent technician tests to the boundary and puts the result in writing. Ring 02 9188 1577 or send us a message.