The discovery of an injured seal believed to have been attacked by a shark has forced the closure of a Perth beach.

Surf Life Saving WA tweeted on Sunday that the Fisheries Department had advised there was an injured seal at Mullaloo Beach, with possible bite marks.

"Council rangers will be closing Mullaloo Beach due to the seal's injuries indicating shark activity in the area," Surf Life Saving WA said.

On Saturday, the Fisheries Department said one of the receivers in the Shark Monitoring Network was out of position, after the buoy to which it was attached broke free of its mooring and was found washed ashore south of Scarborough Beach.

Staff attended the Peasholm Dog Beach to retrieve the equipment and assess the situation.

The Westpac lifesaver helicopter also sighted three whales 400m south of Scarborough, about 250m offshore, tracking north on Sunday.