A man has been attacked by a shark at a popular holiday island. 

The incident happened at Rottnest Island, near Perth, about 10am on Saturday, a St John WA ambulance service spokesperson told AAP.

The man, aged in his 30s, is in a critical condition. 

A rescue helicopter has arrived at the island, 19km off the coast of Fremantle, to evacuate the man to hospital on the mainland. 

The man was transported by boat to shore at Geordie Bay on the north side of the island. 

A five-metre great white shark was spotted 80 metres offshore about the time of the attack, according to Surf Life Saving WA. 

The Department of Primary Industries and Regional Development urged people to take extra caution in the Geordie Bay area. 

Rottnest Island, which measures 11km long by 4.5km wide, is a popular tourist destination.

The incident happened at Rottnest Island, about 10am on Saturday (stock image)

Rottnest Island sits 19km off the coast and is a popular tourist destination