Dozens of swimmers at a triathlon in Western Australia had to be removed from the water after a shark was spotted.

Most of the individual competitors in the Ironman 70.3 in Busselton on Sunday had already completed their 1.9km swim when the shark was seen, but those who remained in the water were removed by Surf Life Saving WA and the beach was closed.

Triathlon WA executive director Peter Minchin said less than 100 swimmers were brought to shore.

The team event, which began about an hour-and-a-half later, was reduced to a duathlon.

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"There was initial frustration but they realised it was in the best interests of their safety," Minchin said.

- AAP