According to data compiled by the Australian Energy Markets Operator, NSW electricity demand will reach close to 13,000 megawatt hours today as millions of heat-struck residents switch on air conditioners and turn up refrigerators and freezers.

This is the highest level since February 2011, when it reached a record 14,744 megawatt hours and compares with yesterday’s peak demand of about 11,000 megawatt hours, recorded late in the afternoon.

However, with much of the state's heavy industry still shut down for the summer holidays, power supply is expected to be uninterrupted.

That could change if bushfires threaten high-voltage transmission lines, which connect the power stations to Sydney's north and west to the city, as happened in Victoria's Black Saturday bushfires of 2009.

The closure last year of one of the state's two aluminium smelters, the Kurri Kurri smelter near Newcastle, has sliced power demand in NSW, leaving a large amount of reserve electricity generation capacity.

According to AEMO data, 14,461 megawatts of power generation capacity is available to meet demand on Tuesday, which will be boosted by potential supplies from Queensland and the Snowy Mountains.

This will leave more than 3250 megawatts of excess capacity to meet any demand spikes or interruptions to supply.

Health authorities have urged residents who need to use air conditioners to do so and not be concerned about the cost or the impact on the network.

Endeavour Energy, which covers the Illawarra and Shoalhaven, said it had cancelled all major planned outages for today, aside from critical safety or repair work, in the interests of customers' comfort and welfare.

"We have extra 'flying squads' of emergency workers ready to undertake network repairs where required," a spokeswoman said.