Rescuers are desperately searching for a swimmer who has been missing at Trigg Beach for several hours.

An alert was issued about 5.15pm Tuesday for the swimmer who police say was in distress before vanishing.

Water police and a police helicopter were at the scene working desperately to locate the individual before the sun went down.

Just before 6pm, rescuers appeared to hone in on a particular area.

A fisherman told The West a siren from the chopper alerted a search boat, which sped over to a certain location.

Two police boats remained in the water once night fell, but the helicopter left after sunset.

Search boats were seen sticking close together while patrolling the water, about 100 metres from the sand.

The second police boat joined the search just after 6pm, the fisherman said.

Anglers were initially asked by police not to fish at the beach at about sunset time, but were eventually told they could continue.

Police at the scene were uncertain whether the search would continue all night. Water police appeared to have left the search area at about 8.30pm.

Beachgoers and members of the public are asked to report any relevant information.