IT'S believed human remains have been discovered in a bag that washed up on a beach on Rottnest Island, in Western Australia.

Police said late yesterday they were almost certain the remains were human, with a pathologist due to be called to confirm the find either last night or first thing this morning.

The grim discovery was made late yesterday afternoon at Porpoise Bay, on the southeast of the island, with police immediately called in to investigate.

Rottnest Island, situated 18km off the Perth coast, is a popular holiday spot.