People planning to set off on Christmas road trips in New South Wales today have been urged to delay their holidays as the state prepares for more "catastrophic" fire conditions on Saturday.

Temperatures are set to soar into the mid-40s and strong winds could make bushfires erratic and difficult to control, the Bureau of Meteorology has warned.

"The peak day for NSW and the ACT is Saturday," forecaster Andrea Peace said.

"It creates a really dangerous period for the fire grounds because it (the wind) rapidly changes the direction of the fires there."

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Sydney is expecting a top temperature of 36C, while the Central Coast is forecast to reach up to 45C. Photo / AP
Sydney is expecting a top temperature of 36C, while the Central Coast is forecast to reach up to 45C. Photo / AP

Sydney is expecting a top of 36C, while the Central Coast is forecast to reach up to 45C. The temperature in Canberra will hit 41C, with smoke haze from nearby fires continuing to blanket the city.

Authorities have warned main highways, including the Princes Highway and Hume Highway, may be closed and motorists will be asked to turn around if they try to enter dangerous areas.

"People should consider staying at home tomorrow and make their travel arrangements for Sunday, Monday and Tuesday," NSW Police deputy commissioner Gary Worboys said.

"They still have time in terms of Christmas Day. Tomorrow will be a difficult day."

Rural Fire Service deputy commissioner Rob Rogers says homeowners shouldn't expect a fire truck, a plane or a warning to save them "because things move really quickly".

"Do not put yourself in harm's way tomorrow," he said on Friday afternoon.

"Do not put yourself near an isolated bush area. If your home is directly on the bush and it's not got a huge break you should think about relocating."

Firefighters are struggling with the conditions in NSW. Photo / AP
Firefighters are struggling with the conditions in NSW. Photo / AP

He also said it was inevitable that part of the Princes Highway on the NSW South Coast will be closed near Ulladulla.

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"We've got problems there with fires to the west that are going to run to the east - we know they're going to impact on the Princes Highway," he said.

There are also concerns the Green Wattle Creek fire southwest of Sydney could cross the Hume Highway with Mr Rogers saying there's a "fair chance" it will be closed.

"We will be doing our best to stop it getting across the Hume Highway because if it gets across ... that opens up a whole new front for us."

NSW Police deputy commissioner Gary Worboys says motorists may be asked to turn around if they try to enter dangerous areas.

"People should consider staying at home tomorrow and make their travel arrangements for Sunday, Monday and Tuesday," he said."They still have time in terms of Christmas Day. Tomorrow will be a difficult day."

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Residents were instead advised to go to the beach, a shopping centre or somewhere else safe.

The horror conditions ahead follow a night of tragedy in South Australia, where devastating bushfires continue to rage through the state's southeast.

Overnight, authorities confirmed one person died when their car slammed into a tree near the scene of a blaze at Lameroo, in the state's southeast.

Catastrophic fire danger is forecast for the Greater Sydney, Illawarra-Shoalhaven and southern ranges regions, while the Greater Hunter and central ranges will experience extreme fire danger.

Meanwhile in Victoria, very high fire danger is forecast for the much of state's north, including in the Mallee, Wimmera, Northern Country and North East districts.

In South Australia, very high fire danger is forecast for the Flinders Ranges, Mount Remarkable, Orroroo Carrieton, Peterborough and Port Augusta.