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."
The utility company is working with the NSW Rural Fire Service to access and inspect parts of the network.
"As soon as we're able to do this, estimated times for when power will be restored will be provided.
"However, access may not be available until tomorrow or Tuesday in some areas."
Hospitals, nursing homes, water stations and petrol stations would be prioritised, Essential Energy said.
Endeavour Energy asked its customers without power to be patient as well.
"The recovery of the network from these mammoth fires is a challenging effort that needs to be undertaken safely and methodically in the days ahead,' the company said in a statement.
Energy Minister Matt Kean on Saturday night confirmed the Australian Energy Market Operator, TransGrid and the NSW and ACT governments were working closely to keep power supplies on.
"We are asking all consumers to be mindful of any unnecessary electricity usage," he said in a statement.
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.
"It (the system) is coping at the moment and we can handle one or two small things happening, but another major loss of a critical asset and we could be in trouble," he told ABC News overnight.
He said if people were "fairly cautious and judicious" with their energy use, "then we could probably avoid some kind of load-shedding event".