Blackout warning as major heatwave to impact millions this week

It's not even summer yet and Sydney is being warned of possible blackouts, with the electricity grid set to be put to the test from tomorrow.

Temperatures will nudge 40 degrees by Wednesday amid a lack of energy supply as critical generators are offline for maintenance.

That means a surge in people switching on air conditioning will occur just as power generation units at three out of four of the state's ageing coal fired power stations are offline for maintenance or repair.

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Officials are warning of "tight electricity supply forecasts" in both states tomorrow and Wednesday.

Premier Chris Minns said he aimsed to let people know what's going on.

"We've had early reports that the grid could be under a lot of pressure. I acknowledge that," he said.

"We will be as quick as possible with updates to the community about potential interruptions with supply."

The Australian Energy Market Operator (AEMO) said it "has alerted the energy industry and is working with power station operators and transmission businesses to boost electricity availability".

Late this afternoon, AEMO advised that energy reserves had improved after transmission outages were cancelled.

But if electricity supply forecasts do deteriorate, it will take all necessary measures.

Energy Minister Penny Sharpe appealed for residents to save electricity.

"On really hot days, do you really need to have every single light on in the house?" she said.

"Do you need to have your air conditioning down at 19 degrees?"

It was 35 degrees at Penrith in Sydney's West by 3pm.

It's set to rise to 38 degrees tomorrow and then a sweltering 39 on Wednesday.

But then the rain will move in.

"If you are across Eastern NSW, unfortunately, you will have to wait until at least Wednesday night and Thursday for relief but that will come in the way of heavy rain and showers and storms from Thursday, Jonathan Howe from the Bureau of Meteorology said.

Temperatures will also drop later in the week but are expected to remain in the high 20s through to the weekend.

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The Bureau of Meteorology has also forecast a high chance of light rain in the city later in the week.

The Bureau has issued a heatwave warning that predicts low-intensity to severe heat for metropolitan Sydney, the Hunter, Illawarra, South Coast, and Southern Tablelands regions.

"Severe heatwaves can be dangerous for many people, especially older people, babies, children, pregnant and breastfeeding women, people with medical conditions and people who are unwell," the Bureau said in its warning.

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"Seek a place to keep cool, such as your home, a library, community centre or shopping centre.

"Close your windows and draw blinds, curtains or awnings early in the day to keep the heat out of your home.

"If available, use fans or air-conditioners to keep cool."

Locations listed as like to be heavily affected included Batemans Bay, Camden, Campbelltown, Hornsby, Liverpool, Nowra, Penrith, Parramatta, Richmond and Scone.

Parts of Queensland's Peninsula District can expect temperatures in the mid to high 30s.

In Queensland, Aurukun, Mapoon, Thursday Island and Weipa will be the hottest.

Brisbane and surrounds will remain a slightly cooler 30 degrees maximum.

The heatwave follows similar weather further south last week, when Victoria and South Australia endured their own scorchers.

Melbourne reached a high of 37 degrees on the weekend before a cool chance brought relief.

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