With Sydney drenched in winter sun, a trip to the beach might be tempting.

But the Bureau of Meteorology has issued a hazardous surf warning for the NSW coast, including advice from emergency services to stay away from the water.

"Surf and swell conditions are expected to be hazardous for coastal activities such as rock fishing, boating, and swimming," the warning said.

Bureau meteorologist Rob Taggart said a low pressure system over New Zealand was producing big swells and rough conditions that have crossed the Tasman.

At 11am on Saturday, a 6.8-metre wave was recorded just off the Sydney coast, the largest since the weather system set in.

"It's not just the height, but the waves actually contain a lot of energy," Mr Taggart said.

He said waves were rolling in very quickly, at 14-second intervals.

The conditions were expected to ease late on Sunday morning, he said.

Maximum temperatures in Sydney are tipped to be well above the July average of 16.4 degrees for each day of the coming week. Sunday is forecast to reach 22 in Sydney, 20 on Monday and Tuesday, and 21 on Wednesday.

"Over the next three or four days we should see clear skies pretty much all day," Weatherzone meteorologist Tom Hough said.

Mr Hough said the warm conditions were a result of westerly winds bringing warm air from the country's interior to the coast.

"That's looking like the set-up for the next couple of days. It's still looking warm into Thursday."

Those clear skies, though, will keep minimum temperatures closer to the long-run July average of just over 8 degrees most mornings. 

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