Sydney and Canberra have broken temperature records that had stood for 80 years as parts of NSW, Australia faced extreme fire danger.

The mercury climbed to 48.9C in the western Sydney suburb of Penrith and 44C in Canberra on Saturday afternoon, the hottest temperatures ever recorded in those areas.

Penrith set a new record for the hottest temperature in the Sydney basin beating the previous mark of 47.8C recorded in Richmond in 1939.

The previous Canberra record was 42.8C at the now-closed Acton observation site in 1939.

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"We've seen many places close to the coast exceed 40C over the southeast and lots of places up on the ranges as well," Bureau of Meteorology acting NSW manager Jane Golding said.

"We've seen many sites in western Sydney above 45C and places quite close to the city above 40C."

Golding said hot and dry northwesterly winds had increased over southeastern NSW on Saturday, before a gusty southerly change started moving up the coast. It's expected to reach Sydney by midnight.

Golding warned there was a heightened risk of undesirable storm activity with a severe thunderstorm warning issued for parts of NSW.

"We are seeing a very unstable atmosphere and there is a heightened risk of storm activity particularly over the far south of the state and the coastal strip up towards Sydney," she said.

BRETT HEMMINGS/ GETTY IMAGES The sky turns red over Bodalla, a small town on the south coast of New South Wales threatened by fire.

"Any storms that develop today, unfortunately, we're not expecting them to bring rain but they would bring those gusty winds and that erratic wind behaviour that's particularly concerning near fire grounds."

The record-breaking heat comes as Australian authorities urge residents in New South Wales not to be complacent because the worst is yet to come.

NSW Premier Gladys Berejiklian on Saturday evening warned people facing extreme bushfires: "We are now in a position where we are saying to people it's not safe to move, it's not safe to leave these areas."

BRETT HEMMINGS/GETTY IMAGES Residents look out over the beach as thick smoke covers the skyline in Tuross Head, New South Wales.

"We are in for a long night and I make no bones about that," she told reporters at the NSW Rural Fire Service headquarters in Sydney.

"We are still yet to hit the worst of it."

The situation is "very volatile" with southerly winds expected to reach up to 80km/h as a change sweeps up the coast from the Victorian border.