That chill includes a forecast overnight low of 9 degrees and a top on Friday of 17 degrees, although the winds will make it feel cooler still.

Snow chance

Snow has already been dusting parts of Victoria, and those falls are now extending to the Snowy Mountains with Thredbo predicting up to 30cm over coming days.

"As that cold pool moves further east, there could be snow down to about 800 metres [on Friday]," Ms Woodhouse said. "That will include the Central Tablelands as well as the Alpine area and the Southern Tablelands."

"It's unlikely we'll see snow around the Blue Mountains," Ms Woodhouse said, adding that it was not out of the question, given places such as Oberon should get a dusting.

Warmth interrupted

On Thursday, Sydney reached almost 26 degrees ahead of the cold front's arrival.

Daytime temperatures so far in May are running more than 5 degrees above average. Minimums are about 2 degrees above the long-term norm.

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The record of unseasonable warmth will be interrupted for a few days, although daytime temperatures are forecast to climb back into the low 20s for Sydney by the middle of next week.

While the drop in temperatures from Friday into the weekend will probably be enough to put people off having an ocean swim, the prospect of big waves will make sea activities potentially dangerous along much of the NSW coast.

The size of the swells will be "highly dependent on where that low-pressure centre is", Ms Woodhouse said.

Current forecasts of significant wave heights along the NSW coast of two to three metres - and four to five metres offshore - may be raised if the low drifts further north than currently forecast.

Surf could be "dangerous if not damaging" if the winds turn gale force offshore, she said.