Waves of up to 18 metres in height are expected to hammer the Sydney to Hobart fleet by Tuesday in what looms as the worst conditions since the 1998 tragedy.

Australian Bureau of Meteorology forecaster Peter Dunda has told the 117-strong fleet that conditions will "deteriorate rapidly" on Monday as they sail down the NSW South Coast.

"By Tuesday...it will be very cold at sea, and the southwesterlies will reach 40 to 50 knots, sometimes 60 knots in squalls that could contain hailstones," Dunda said on the official race website.

"It is likely to be so cold there could be snow in the Tasmanian highlands and in the Alps on the mainland.

"At the peak of the gale, wave heights may average six to nine metres, with the possibility that some waves will reach twice that size."

The race is expected to start on Boxing Day in "fairly good conditions" with light to moderate south easterly winds in the morning giving way to 10 to 15 knot noreasterlies in the afternoon.

Dunda forecast strengthening north to norwesterly winds on Monday morning ahead of a front which was expected to deliver gale force south westerly winds to the NSW far south coast during the morning.

He predicted the wind would increase in strength from 35 to 40 knots by the evening and would be even stronger on Tuesday night when rough seas and heavy swells were forecast.

While the bigger boats could complete a significant portion of the race before the bad weather kicked in, there was little cheer for the smaller yachts with the possibility of gale force winds continuing until Thursday.

The forecast significantly improved the prospect of the first line honours-handicap winner of the Sydney to Hobart in 17 years.

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