Sweltering workers and residents are being asked to go easy on air-conditioning and turn off lights and appliances as authorities scramble to prevent blackouts amid a late spring heatwave.

The national energy regulator has forecast a shortfall in electricity supplies in NSW between 3.30-5pm on Wednesday, brought on by high demand and outages at key coal-fired power plants.

Hundreds of thousands of public servants across NSW were asked to lift thermostat settings on air-conditioners and take other energy-saving measures.

Major energy users such as water providers and the City of Sydney will lower their power demands between 3-8pm, shifting electricity usage outside the peak window.

But NSW Energy Minister Penny Sharpe called on the public to also do their bit to ensure there were no power shortages.

“It’s really important for people to know this is voluntary, people have situations where they are not able to do that, that is OK,” she said.

“But what we are trying to do is reduce demand across the system by large energy users, by the government as well, and we now ask households and businesses to see what they can do … a lot of small activities actually really help demand across the network.”

The Australian Energy Market Operator’s declaration urged industry to generate as much power as possible and restore all available powerlines across the grid to meet the rocketing demand.

The Bureau of Meteorology has issued severe heatwave warnings for large swathes of the NSW coast, including the Sydney metropolitan area, the Hunter region and the south coast.

There are also warnings for parts of the ACT and Queensland, including an extreme heatwave for Australia’s far north.

Elsewhere, Victorian health authorities issued a warning for thunderstorm asthma in the state’s north-central and West and South Gippsland regions.

“The combination of forecast high grass pollen levels and a certain type of thunderstorm with strong winds means that there is a chance that a large number of people may develop asthma symptoms over a short period of time,” they said.

“Health and emergency services are monitoring the situation closely and are ready to respond.”

Four coal-plant units are offline in NSW, with some operators carrying out planned maintenance before the usual summer peak in demand.

But climate change had extended the hot season, which should prompt a re-think on when maintenance is done while renewable energy sources are rolled out, Premier Chris Minns said.

“Utilities go through their maintenance program and repair program so they’re ready for the peak summer period or the peak winter period,” he said.

“[But] increasingly, you’ll see heatwave conditions earlier and earlier in the year and obviously we have to manage that, but at the same time, we’re seeing more solar come on and more battery technology that can fill the gap.”

AEMO continues to work with industry & government to manage expected low electricity reserves this afternoon in NSW.

To ensure supply meets demand, AEMO has activated all available resources & secured additional reserves through the RERT mechanism. https://t.co/PlXfj8DSSc pic.twitter.com/cs6eMrwwCe

— AEMO (@AEMO_Energy) November 27, 2024

Federal Energy Minister Chris Bowen earlier insisted there was enough electricity in reserve to get through the day, adding the system was acting as intended.

“Obviously, AEMO is always on the lookout for unexpected events which they need to prepare for, they’ve put all the necessary protections in place as they can at this point,” he said.

Sydney’s city centre rose to nearly 33 degrees late on Wednesday morning, while by early afternoon the mercury had risen to nearly 39 degrees across a swathe of suburbs from the south-west to north-west.

Western Sydney peak temperature forecasts were more than 10 degrees above November average maximums, weather bureau senior meteorologist Miriam Bradbury said.

“While anomalously high, these temperatures are not expected to break any records… It’s more the duration of the warm conditions, including those warm nights, that is more significant,” she said.

“It’s also worth noting that the ‘feels like’ temperatures will be oppressive due to the high humidity levels.”

Rain is predicted to hit eastern NSW on Thursday, although warm, humid conditions will persist.

-with AAP