A stunning hat-trick of victories has given Australian stars Malcolm Page and Mathew Belcher a huge boost at the ISAF Sailing World Championships in Fremantle.

On a day when race organisers were on alert after a four-metre shark was spotted in the area, Belcher and Page displayed killer instincts to win all three of Wednesday's 470 class races in 20 knot winds and large swell.

The world No.1-ranked pair led the first fleet race from start to finish and ended 30 seconds ahead of French duo Nicolas Charbonnier and Vincent Garos.

They took out the second race 29 seconds ahead of second placed British crew Nick Rogers and Chris Grube and then won the third race ahead of another British crew Nic Asher and Elliott Willis.

While scores were still to be calculated, the three victories are expected to leave Belcher and Page first or second on the regatta leaderboard with plenty of racing still to come.

Australia's triple Olympian sailboarder Jessica Crisp had another mixed day in the RS:X fleet, finishing 16th and ninth in her first two races.

Former world champion Krystal Weir finished 17th and fifth in the day's Laser Radial races.

Earlier on Wednesday, event director John Longley ordered extra precautions, including aerial patrols, after a fisherman reported seeing a great white shark near the competition area on Tuesday evening.

"The report was made by a professional fisherman who is regarded as a reliable source," said Longley.

No races were delayed.