Perth's shark patrol helicopter took off for its first flight of the season on Saturday morning - as a great white menaced City Beach and Floreat Beach and a bronze whaler shark was detected off Warnbro Sound.

The great white shark was estimated to be about 3.5 meters long and was swimming south. City and Floreat beaches were both closed.

The Westpac helicopter has started shark patrols along metropolitan beaches.

The Westpac Lifesaver Rescue chopper is based at Rouse Head in North Fremantle and will patrol from Yanchep to Mandurah and out to Rottnest.

It will fly only at weekends until October 1 when it will operate seven days a week.