NSW energy provider Ausgrid says it is "never going to be able" to supply all of its customers all of the time due to how much it would cost.

A day after 31,000 properties across NSW lost power in searing heat, Ausgrid system control head Stuart Donaldson said the community would not want, or be able to pay for, continuous coverage.

"We're never going to be able to supply 100 per cent of customers 100 per cent of the time," Mr Donaldson told reporters in Sydney on Monday.

"The community doesn't want to pay the amount of money, or can't afford to pay the amount of money, that it would cost to do that."

Mr Donaldson, who apologised to customers for the blackouts, said there were 42 major outages across the state on Sunday.

Just four of the outages were caused by overloads on the network.

Others were due to a variety of issues, according to Mr Donaldson, including underground cable failures.

"What we have to do is be prudent in how we spend our customers' money and make sure we get the most efficient outcome for them."

NSW Opposition Leader Luke Foley on Monday took aim at the government for failing to diversify its energy base.

"This state government, alone of all Australian state and territory governments, hasn't prepared for heatwave conditions," Mr Foley told reporters.

"Most of the other governments have delivered new storage and new energy capacity."

Mr Foley said it was lucky the extreme heat that Sydney experienced on Sunday, with some areas exceeding 47C, occurred on a weekend when much of industry was not using energy.

"The government's only plan seems to be that heatwaves occur on weekends," he said.

NSW Energy Minister Don Harwin said he believed the state coped "exceptionally well" for such a hot day.

"Localised outages are par for the course with electricity networks throughout the world - especially on days of extreme heat like experienced yesterday," Mr Harwin told AAP in a statement.

"We expect Ausgrid to meet its strict reliability standards."