The change from Telstra ADSL to NBN VDSL drops the POTS line that gave telephone access independent of premises power.
Telephony is now provided by an ATA integrated into the VDSL modem, and dependent on mains power.
NBN makes no definitive statements about battery backup to the node, or endurance of any battery backup. Nevertheless it appears from pics people have taken of the node cabinets that the bottom layer is batteries and it is likely that they have endurance of more than 10 hours.
Above, a pic of an NBN FTTN cabinet from NBN's website, and showing batteries in the bottom layer.
NBN is not customer facing, and it seems they will provide are reluctant to provide all manner of information useful to end users, perhaps guided by lawyers who do not want to create any obligation or liability for the company… the way of modern telecommunications.
In that context, it would seem worth the risk to provide battery backup to the VDSL modem to provide broadband access to battery devices such a tablets, laptops and smartphones, and access for a POTS handset plugged into the ATA port.
The supplied modem includes a 12V DC power pack rated at 2A.
Above, measurement of the current drawn averages around 0.5A with all features running (though it may draw more current during phone ringing).
It would seem that a common 7Ah gel cell could directly power the modem and would provide endurance of more than 10h. So for a cost of under $200 for a battery backed power supply using a common 7Ah gel cell, we get some protection against premises mains failure and possibly area mains failure depending on the node endurance.
Above is an example battery backed supply intended to house a 7Ah gel cell, and featuring a charger and low voltage protection of the battery.
The project awaits proving of the modem. I have never found the modem supplied by an ISP to be sufficiently capable, this one might be a surprise and for simplicity it would be great if it was sufficient… but it has locked up a few times where I cannot get access to the management console without rebooting it, a process that takes about 5min and disrupts everything using the Internet.