The communications watchdog will force telcos to warn National Broadband Network customers about the risk of their phone lines being disconnected during blackouts.

Unlike legacy copper phone lines, which carry electricity and keep phones powered during an outage, high-speed fibre lines cannot carry a charge.

Devices connected to fibre cabling are instead powered by a house's mains, meaning they become useless during a power outage in the absence of a back-up power supply.

Disconnection could be especially problematic for older people, who still largely rely on their home phones.

Medical alarms and other services running on phone lines are also at risk of disconnection.

Australian Communications and Media Authority chairman Chris Chapman says fibre consumers must be told about risk so they can decide whether to install a battery back-up.

"Without reliable safeguards to ensure adequate risk information is provided to inform that choice, there is a small but real chance that unaware consumers may be put at risk, particularly in an emergency."

Back-up battery units have been available for installation on an opt-in basis since December.

They're free to customers who have life-threatening medical conditions, supplying about five hours of charge during a blackout.

The warning affects the quarter of NBN customers who will have fibre cabling connected directly to their homes or businesses.

Customers on fibre-to-the-node connections are not affected.

Mr Chapman said he was confident the new arrangement, which comes into force in October, would not unnecessarily burden the industry.