The men were near Castle Rock late Saturday afternoon when a shark targeted their inflatable boat, puncturing it.

WA Fisheries urged water users to exercise caution at Dunsborough following the attack.

"Beaches have been closed at both Castle Rock and Meelup and will remain closed until mid-morning Sunday when Fisheries and the City of Busselton reassess the situation," a spokesperson said.

"A Fisheries boat will be on the water monitoring the area, and the Surf Life Saving WA Westpac Lifesaver Rescue Helicopter will also conduct a sweep of the area at the start of today’s patrols." 

The attack is the second in Western Australia this month, after Sean Pollard lost both hands and part of an arm near Esperance.