Just when you thought it was safe to go back in the water, another shark has reared its fin at St Kilda beach.

About 8pm on Monday, the Vic Emergency website said the beach had been closed following a confirmed sighting of a small shark in waters close to the beach.

Life Saving Victoria general manager Paul Shannon said extra lifesaving patrols had been volunteering at the beach due to the warm night.

They had confirmed a sighting of a shark and recommended people didn't swim at the beach for at least two hours, after which time the situation would be reassessed, he said.