A widespread power failure would impact on some hospital services, but Parliament House and defence buildings would continue to operate largely as usual.

The nation's northern-most capital shut down on Wednesday, with schoolchildren and public servants directed to stay at home, after a city-wide "code black" which was caused by a tripped circuit at a sub-station - and safety shutdown at another station - shortly after 1am.

The blackout stretched to Katherine some 320 kilometres away as Darwin's CBD left without power for 12 hours.

ActewAGL asset strategy and planning manager Dennis Stanley said Canberra's network design would need failures at far more locations for the entire city to be plunged into darkness.

"Within the ACT, ActewAGL owns and operates 14 transmission zone sub-stations and switching stations with seven circuits that connect to the NSW transmission grid," Mr Stanley said. "All seven circuits would need to have failed. The physical design and ringed nature means it is unlikely we'd have that problem."

Mr Stanley confirmed a city-wide blackout would impact on traffic lights, schools and most public service workplaces, as in Darwin.

A widespread, extended blackout would not stop the nation's politicians, with Parliament House equipped for the contingency.

"Under those circumstances Parliament House will be affected, but it has back-up systems that will allow limited functionality for up to several days," a spokeswoman said.

"The Department of Parliamentary Services has standby power generation for the building, for emergency and related support systems, with additional battery back-up to our computer suites should a power outage occur."

The Defence Department said its sites around the city could also supply their own power, with "appropriate mitigation strategies in place" for critical facilities.

"Defence has emergency generators to manage power failures or blackouts throughout Defence establishments and bases in Canberra," it said in a statement.

An ACT Emergency Services Agency spokesman said energy emergency plans were in place, and ActewAGL could direct available power supplies selectively to priority sites.

ACT Health said a city-wide blackout would have impacts for Canberra Hospital, but back-up generators would supply power to essential services, such as the clinical areas of the hospital and support services such as lifts.

Mr Stanley said the city's worst modern power failure was caused by the 2003 bushfires, when it took nine days to replace 960 poles and return power to all 37,000 affected customers.

Canberra's population of about 385,000 is three times that of Darwin, and the city is home to about 40 per cent of the nation's federal public servants.

The Northern Territory government has said a review into Wednesday's failure would be carried out in the next fortnight.