Sydney - Four supermaxis were in a battle for glory in the epic Sydney to Hobart bluewater classic down Australia's east coast on Thursday, in what organisers said was the closest contest in the history of the race.

The lead has been switching between 100-footer defending line honours champion Comanche and fierce rival Wild Oats XI since a fleet of yachts departed a sunny Sydney Harbour on Wednesday at the start of the 628-nautical mile (1 163km) race.

Keeping touch with them are the two other mega-yachts - Black Jack and Infotrack - with just five nautical miles separating all four.

The yachts were off the record pace of one day, nine hours, 15 minutes and 24 seconds set by Comanche last year, amid lighter winds.

"This is the closest race we've been in - we've been in close races with Comanche before, but never been in such a close race with four of us," Wild Oats XI crewman Chris Links said early Thursday from the yacht.

His boat was first to Hobart in 2017 but was stripped of the win after being handed a one-hour penalty over a near-collision.

"We crossed tacks with Comanche a couple of times this morning between 03:00 and 04:00. We can just see Black Jack too. There's nothing in it," Links added.

The boats were entering the notoriously wild Bass Strait between Tasmania and the Australian mainland on Thursday, with forecasts tipping the region to serve up erratic conditions including windless holes.

TP52 Ichi Ban - last year's handicap honours winner - is leading the race to be the vessel that performs best according to size.

Chasing them for the handicap Tattersall Cup trophy is 40-footer Chutzpah and 60-footer Winning Appliances, which has Hawaiian two-time world surfing champion John John Florence on board.

"Day two (Thursday) is going to be a big one. The wind is dying," the Commodore of organiser the Cruising Yacht Club of Australia, Paul Billingham, told national broadcaster ABC.

He added that it was a "cat and mouse game" between the supermaxis, and also a close contest for the handicap prize.

"The handicap lead is changing every five minutes. There's probably 10 boats that are within an hour of each other on corrected time."

The fleet in the gruelling contest is down to 81 from 85, after Hong Kong supermaxi Scallywag retired early with a broken bowspirit, followed by Zen, Patriot and M3 Team Hungary.

The crews will mark the 20th anniversary of the 1998 tragedy with a moment of silence and the reading out of a commemoration message later on Thursday.