Perth skipper ‘didn’t notice’ three passengers had been flung overboard

Three people are lucky to be alive after being lost in rough seas off Perth for close to three hours Saturday night.

The trio were returning from a Raft Up Party on Rottnest Island when they were thrown from the back of their 6.4m inflatable vessel just after 8pm.

The boat's skipper noticed minutes later, but by then the group had been lost, prompting a frantic search involving water police and Fremantle Sea rescue.

It was an oilrig support vessel, the Far Statesman, that eventually found the trio – almost three hours later.

"We actually heard shouts in the water off the bow, about 200 metres away and as we approached we picked up the three people we could see in the searchlights," Captain Robert O'Dowd told 9NEWS.

By this point the three had drifted almost two kilometres from where they are thought to have fallen in, braving extremely rough conditions, with winds of up to 35 knots.

The hapless group were brought aboard a police rescue boat shortly after and taken to hospital, where they were kept overnight while being treated for exhaustion.

Police said the trio were fortunate to have survived the ordeal.

"Given the weather conditions last night and also it being at night – extremely lucky," water police Acting Sergeant Alan Trist said.