Staffing levels at Ausgrid have fallen below minimum legal levels, which is hindering the electricity supplier's efforts to restore power across NSW, the industry's union says.

Key points: Under the terms of its privatisation Ausgrid must have at least 3,570 full-time staff

Under the terms of its privatisation Ausgrid must have at least 3,570 full-time staff Ausgrid says its levels are still above the legal requirement when contractors are counted

Ausgrid says its levels are still above the legal requirement when contractors are counted More than 20,000 homes and businesses remain without power following storms

A leaked internal document, obtained by the ABC, shows Ausgrid's total number of staff as of December 2019 was 3,238 — 10 per cent below the minimum legal requirement.

Under the terms of its partial privatisation by the NSW Government in 2015, Ausgrid is legally required to maintain at least 3,570 full-time equivalent staff.

Electrical Trades Union Secretary Justin Page said it was clear staffing levels were inadequate and were affecting Ausgrid's ability to restore power supplies after last week's storms.

"Ausgrid does not have the resources to adequately restore power when these natural weather events occur," he said.

"These distribution workers have been working for months now trying to restore power through fires, floods, storms, under-resourced and working long hard hours and frustrated that they can't provide the service that they used to provide to the community to restore power quickly."

Ausgrid's chief operating officer Trevor Armstrong said staff numbers were above the legal requirement and the leaked document did not account for long-term contractors employed for more than 12 months.

"Ausgrid's staffing levels are above the legislated level of 3,570," he said.

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He said Ausgrid's latest staff levels, as reported to the Independent Pricing and Regulatory Tribunal in February were actually 3,950.

Mr Armstrong also disputed suggestions Ausgrid was inadequately staffed to respond to the storm.

"No good business would resource up to the level required to meet this sort of natural disaster," he said.

"That's why we have sharing arrangements in place throughout the country for electrical resources in this time of need, which we've put in place."

Ausgrid said almost 19,000 homes and businesses were still without power in Sydney and the Central Coast.

It has enlisted additional crews from Energy Queensland and around 150 staff from the State Emergency Service, Rural Fire Service and Fire and Rescue NSW to help restore services.

NSW Premier Gladys Berejiklian said it was appropriate for Ausgrid to call in extra resources.

"This was a once-in-30-year event," she said.

"As soon as the Government was made aware that Ausgrid was struggling we've made sure those extra people have gone in and we're making sure that power is restored as soon as possible."

Ausgrid said it hoped to have power restored to all of its customers by Sunday.

About 50 Endeavour Energy customers in the Hawkesbury area are still without power but the company said it hoped to have their services restored by this evening.

Labor's shadow minister for climate change and energy Adam Searle said Ms Berejiklian made a commitment in law that Ausgrid would maintain minimum staffing levels to ensure proper service to the community.

"This is nothing short of a scandal. These documents have shown that Ausgrid has cut more staff than they were allowed to.

"No wonder Ausgrid has struggled to reconnect communities across the network to communities in need."

NSW Minister for Energy and Environment, Matt Kean said the storm was one of the biggest to hit the network in 20 years with more than 180 poles and 1,000 cables down.

"I want to apologise to those that are still without power and let them know that we're doing everything possible to get them on," he said.

"As soon as Ausgrid asked us for additional resources I responded immediately."