The Emergency Services Agency said electricity demand had significantly eased with cooler weather and there was now a considerable reserve in the NSW/ACT region.

"Despite this, Canberrans are asked to be conscious of their electricity consumption during periods of extreme weather and to turn off appliances if they are not needed," the agency said in a statement.

More than 40,000 customers remain without power in southern NSW after transmission lines were destroyed by bushfires.

Essential Energy said 35,600 users did not have power in southeastern NSW on Sunday and were unlikely to regain supply for days.

Some 6500 Endeavour Energy customers were also without power mainly in the southern Shoalhaven region.

"The electricity network has sustained extensive damage with some areas being completely destroyed and needing rebuilding," Essential Energy said in a statement.

"In many cases customers will be without power into late this week, some possibly longer."

TransGrid chief executive Paul Italiano said assets in the Kosciuszko National Park had been damaged, interrupting the supply between Victoria and NSW.

Mr Italiano said whether there would need to be load shedding - or rolling blackouts - depended on further losses.

On Saturday night, the Environment, Planning and Sustainable Development Directorate said the extensive bushfire activity in the Snowy Mountains and other parts of New South Wales had impacted electricity supplies.

"The Australian Energy Market Operator, TransGrid and the NSW and ACT governments are working closely to keep power supplies on, but due to today's weather conditions as we approach the evening peak there may be a need to turn off power in parts of the network to keep the overall system secure," the Emergency Services Agency said in a statement.

The Australian National University said it expected power outages at its Acton campus on Saturday night and warned residents to prepare for intermittent outages.

The Australian Energy Market Operator is also working with large load electricity customers to reduce their electricity consumption where possible.

NSW Rural Fire Service Commissioner Shane Fitzsimmons said high voltage lines, connecter lines and the switching hubs between the Tumut and the Upper Tumut areas had been impacted by the fire

"We are working with the power companies right now about what is the impact and what needs re-energising," he told the ABC.

READ MORE:

"The challenge with re-energising is what we don't want to do is cause electrocution to firefighters working in and around that area, particularly where there might be fairly intense fire activity under those main feeder lines.

"So we are working through a process at the moment with the authorities to identify what is a level of impact, and any disruption that may be occasioned as a result of that. And if we need to re-energise lines, we need to make sure that firefighter safety is the highest priority and we need to make sure that no-one is in the vicinity of those high voltage lines and any existing activity should we take that procedure.

"I should say, too, across those fire zones we are getting reports of significant damage and destruction that are starting to come in with reports of property impact and properties burning in a number of these different fireground locations, given the speed and ferocity at which these fires are burning, and I think we do, unfortunately, need to be ready, probably tomorrow morning, once we start getting an indication of what that level of damage and impact might have been throughout this afternoon and, indeed, over the coming hours into this evening."