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Penrith, near the foothills of the Blue Mountains, can expect maximum temperatures of 36, 37, 41 and 41 degrees in the four days from Friday, the bureau said.

Areas of the Riverina will most likely climb above 45 degrees on Sunday and Monday as the heatwave laps eastwards. Those seeking relief by the beach may not find a lot on the South Coast, with places such as Nowra nearing 40 degrees on both days, and Bega heading towards 42 degrees by Monday.

Ms Sturm said the heat may begin to break down by Australia Day on Tuesday and some showers will be present but also the possibility of thunderstorms.

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The last break before school and work resumes for many "will make for an interesting weekend", Ms Sturm said, noting the COVID-19 restrictions on gatherings.

Fire danger ratings will be on the rise. The Riverina is among the areas with "very high" risks by Friday.

A large high pressure system in the Tasman Sea with an associated ridge stretching across northern NSW and into Queensland is the immediate driver of the heat, Ms Sturm said.

A low-pressure system to the south, though, is stirring up a big swell ahead of the arrival of the warmth.

The bureau has issued a hazardous surf warning for Tuesday for the NSW coast from Coffs Harbour down to the Victorian border. On Wednesday, the warning for rock fishing, boating and swimming will extend along the entire coast.

Along with the warmth, most of NSW will be dry for the next week or so, the bureau forecasts.