Hundreds of Ironman swimmers have been stopped from entering the water after a shark was spotted off the Busselton coast on Sunday morning.
The 2??-metre shark was first spotted by a Surf Life Saving WA helicopter at 8.25am today, an hour and 25 minutes after the race began at 7am.
It is understood some swimmers were still in the water when the shark was spotted to the east of the course, inside the bay.
Cont - shark sighted 200m offshore , 100m east. Beaches closed & water cleared. Busselton Jetty, Busselton. Reported 11:50hrs 07/05??? Surf Life Saving WA (@SLSWA) May 7, 2017
Commentator Simon Beaumont said he was calling the swimming leg of the race, when the surf lifesavers acted.
"All of a sudden we could see all the boats, the rescue craft and surf lifesavers come charging towards us with a whole pile of people on board - a whole pile of pink hats," he said.
"They probably pulled about 80 or 100 people out of the water and I think people picked it almost straight away that something had happened.
"The decision was made by water safety to get the remaining swimmers out of the water ... from there, the decision then had to be made about the team event which was due to start about a hour and a half later, and the City of Busselton and race organisers chose to keep the beach closed - so no team swim.
"It was only a small shark, it didn't interact with swimmers ... I guess common sense prevailed but we've got a few disappointed swimmers down here."
Triathlon WA director Peter Mischin said the surf lifesavers enacted the emergency evacuation procedure immediately after the helicopter gave its signal.
"It was actually towards the end of the swim leg, so there were only a few individual swimmers left in the water.
"The shark was spotted a fair way from the shore, but the Busselton surf lifesavers - who provide water safety at the competition - evacuated the water quickly."
The beach has since been closed.
Mr Mischin said Triathlon WA would convene later to discuss how the aborted swim leg would impact future events, as Busselton 70.3 is considered a qualifier for a number of other international races.
On Saturday evening, Triathlon WA also warned competitors the race would be wetsuit-only, as the water temperature was below average.
We would like to welcome all of the #BusseltonIronman competitors to our gorgeous City - best of luck this weekend! #EventsCapitalWApic.twitter.com/P47uAIcmLp??? City of Busselton (@CityBusselton) May 5, 2017