More than 100 yachts set off on the annual Sydney-Hobart race Tuesday, with storms and hail threatening to lash sailors in one of the world's most gruelling ocean events.

Thunder and rain lifted just minutes before the start at a slightly misty Sydney Harbour as spectators lined the waterfront or crowded onto boats to get a better view of the dramatic opening moments.

At the sound of the starting gun, 103 yachts tacked in light winds in search of the best line out of the harbour and an early lead in the punishing, 628-nautical-mile (1,200-kilometre) crossing to the Tasmanian state capital.

A little over two hours into the race, 100-foot supermaxi LawConnect -- runner-up in the three previous editions -- was in front, according to a live online tracker and race organisers.

Not far behind in second place was the favourite and last year's winner, fellow supermaxi Andoo Comanche, which holds the 2017 race record of one day, 9 hours, 15min and 24sec.

SHK Scallywag, another supermaxi, which sports a taller mast this year after undergoing extensive modifications, was in third place.

"We back our boat in any conditions to win the race," Andoo Comanche skipper John Winning told a news conference ahead of the start.

While preferring weather that favours the quickest possible race, Winning insisted his yacht was "fast in all conditions".

LawConnect's crew, though, was getting fed up with second-place.

"I always want to win the race and to actually, finally win it would be a dream come true," LawConnect skipper Christian Beck told Channel Nine news ahead of the race.

"The start is awesome and the pub at the end is awesome, so I like the whole event."

Weather is pivotal in the blue water classic, first held in 1945.

This year marks the 25th anniversary of a fatal storm that tore into the 1998 race, whipping up mountainous seas and wild winds in which six men died, five boats sank and 55 sailors were rescued.

For the 2023 edition, weather forecasters are predicting storms, lightning, hail and strong winds as the boats sail southwards down the New South Wales coast on their way to Hobart.

Sailors face a risk of "severe" thunderstorms within hours of the start, said the Bureau of Meteorology's senior meteorologist Gabrielle Woodhouse.

In a final briefing on race morning, she predicted easterly but "very erratic" winds in the early stages as a low pressure system creates strong winds, heavy downpours and possible hail, hindering visibility.

Vessels still on the water Friday could face a southeasterly swell with waves of three-to-five metres (10-16 feet), she said.

David Witt, skipper of the Hong Kong-owned SHK Scallywag, said the forecast looked "very exciting" for his crew.

"So it's all or nothing for the Scallywags yet again," he said in a social media message to supporters before the race.

Last year, Andoo Comanche crossed the finish line first with a time of one day, 11 hours, 56min and 48sec.

But the overall winner of the race under a handicap system was 52-foot Celestial, which claimed the coveted Tattersall Cup.

Another 52-footer, Caro, and the 72-foot URM Group, are also among the favourites for overall race honours this year.

Ten international yachts are competing, with three from New Zealand (Allegresse, Caro and Niksen), two from Hong Kong (Antipodes and SHK Scallywag) and other entrants from France (Teasing Machine), Germany (Rockall 8), Ireland (Cinnamon Girl - Eden Capital), New Caledonia (Eye Candy) and the United States (Lenny).

Nine-time line honours-winning supermaxi Wild Oats XI did not enter this year. (AFP)