A passenger miraculously survived when a light plane nosedived into the sea off Sydney after onlookers plunged into the choppy ocean to rescue him.

The pilot was killed in the crash just off a northern Sydney beach despite two policemen stripping off their uniforms and swimming to the wreckage to try and find him.

The aircraft sent a mayday alert before coming down some 50 metres (165 feet) off Curl Curl beach in strong winds. It could be seen on television pictures lying on the ocean bed.

An eyewitness, who gave his name as Travis, told reporters he was working on a nearby building site when he noticed the plane circle overhead before pitching into the ocean.


"Me and another guy ... we ran out, swam out, we pulled one guy out, the passenger,'' he told local radio.

"We got to the aircraft but (the passenger) had already popped out, he was sort of floating. He couldn't swim, he'd hurt his back pretty bad so we paddled him over to the shore.''

Television footage showed the man being stabilised on the rocks before being winched into a rescue helicopter and taken to hospital with spinal injuries.

Constable Jacob Gow and Chief Inspector Colin Green were among the first on the scene and dived into the ocean to check on the wreckage.

"Jacob and I ... very quickly, without even thinking, got partially undressed and then swam out to the plane,'' Green told reporters.

"There was a person in the plane but due to the depth of the water and not having any diving equipment, we weren't able to access inside the plane.''

Superintendent Doreen Cruickshank said the aircraft appeared to get into difficulty as it was flying north across the beach.

"It nosedived into the water, it stayed on the surface for about two minutes and then sank,'' she said.