Sydney's April and the first four months of the year were the hottest on record for those periods, according to Bureau of Meteorology temperature records going back to 1859.

Wild weather

Come Friday, though, the mercury will struggle to reach 17 degrees even without the wind chill. The day will be the coolest, on current forecasts, since last November.

"It will be quite a shock to the system," Mr Brittain said.

Little more than showers are likely, though, until Sunday when falls of 1-8 millimetres are expected for Sydney, the Bureau of Meteorology said.

However, Sunday could be more notable for the strong winds, as the second trough associated with the big low moves across the region. Exposed parts of the Sydney coastal region, such as Cronulla and Wattamolla, may be hit by winds of up to 90km/h, Mr Brittan said.

Snow falls are possible across the ranges, including in Oberon to the west of the Blue Mountains, he said.

According to Weatherzone, Melbourne's tops of 13 on Thursday and Friday would be that city's coldest pair of days so early in the season since 1978.

For Canberra, Friday's top of 9 degrees would be the national capital's coldest this early since 1970, Weatherzone said.

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The city also posted its hottest April on record, so residents might also find the change abrupt.

Big waves

Along with the strong winds, powerful waves are likely to hammer the country's southern and eastern coastlines.

"Significant wave heights are forecast to reach five to seven metres in waters near exposed coasts of central and eastern South Australia, western Victoria and Tasmania from Thursday into Friday," Ben Domensino, a Weatherzone meteorologist, said.

"Maximum wave heights will be larger than this and beaches will be dangerous for activities like surfing."

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