Energy Minister Matt Kean said the grid under come under strain owing to recent extreme weather conditions and the impact of bushfires on the state's transmission lines.

"The peak period for power use in NSW is expected to be between 4pm and 8pm," Mr Kean said. "We are asking consumers to reduce their demand during this period where it is possible and safe to do so."

Mr Kean suggested consumers close doors, windows and blinds to keep the heat out, switch off non-essential appliances such as pool pumps, cool a minimum number of rooms, and set air conditioners to 26 degrees.

"During a similar heatwave incident earlier this year a fantastic consumer response went a long way to reducing the load on the grid. We are again asking everyone in the community to play their part in reducing pressure on the national grid," he said.

Temperatures reached 40 degrees in Newcastle on Thursday afternoon, according to weatherzone, as the Bureau of Meteorology warned of possibly severe thunderstorms in the Upper Hunter and New England.

Storms were likely to produce damaging winds at Scone, Armidale, Tamworth, Gunnedah, Moree and Narrabri, the Bureau warned around 3pm.

In Newcastle, there was a slight chance of a thunderstorm under gusty northwesterly winds up to 40 kilometres.

Mr Kean said the government had activated its energy action response protocol on Thursday afternoon, which he said could "potentially reduce demand equivalent to more than 24,000 households".

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