The Narrabri parents of a man reported missing from a Western Australian beach have arrived in Perth to be briefed by police.

Martin Tann, 24, was last seen on Tuesday after telling relatives he was going to Mullaloo Beach, about 30km north of the Perth CBD.

Mr Tann's backpack was found by a passer-by on a limestone wall near the beach, while his wallet and clothes were spotted on the sand.

An extensive land and sea search has so far failed to find any trace of the young marine biologist.

WA Police say they are also checking if any possible sightings of Mr Tann have been reported to Crime Stoppers, and are appealing for public assistance to help locate him.

A witness, who came forward on Wednesday, has told police he saw the 24-year-old swimming in waist-deep water shortly before he was reported missing.

SES volunteers and surf lifesavers have also assisted police in the search.

Police believe Mr Tann entered the water, but don't know his movements beyond that.

Inspector Geoff Desanges says police are still considering a wide range of possibilities.

"It's a big ocean out there, with a lot of area and a lot of possibilities, and we're certainly not suggesting the only option we're considering is that Mr Tann entered the water and didn't come out," he said.

"We're exploring a series of other options, land based ones also."