ABC Aerial surveillance of Perth beaches has started for the summer season.

Two Perth beaches have been re-opened after the first shark sighting of the season, a 3.5-metre white shark 50 metres off Floreat beach.

Surf Life Saving WA said the shark was tracking south, and that Floreat Beach and City Beach were closed as a precaution.

The sighting comes as weekend helicopter patrols begin along the Perth coast for the summer season.

The Surf Life Saving helicopter will fly between Dawesville and Two Rocks looking for swimmers in trouble, and for sharks close to shore.

SLSWA's manager of aviation services Peter Scott said it is a busy time of the year for shark sightings, with a lot of whales and other marine animals in the area.

"We can provide some assurance and also we use the aircraft to verify sightings from the public as well, so we can try and identify if there is actually a shark there," he said.

"But essentially it does provide that extra layer of comfort for everyone."

WA Premier Colin Barnett said the patrols would increase to seven days a week from October 1.

"SLSWA plays a crucial role in protecting Western Australian beaches and it is vitally important to have the aerial patrols in place in time for the warmer weather," Mr Barnett said.

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Perth beaches re-open after sighting of 3.5-metre shark on first day of aerial patrols