There have been at least three reports of shark sightings in the past three days, including at Brighton Beach on Tuesday, Hampton on Sunday and St Kilda Beach on Monday. 

There was an unconfirmed shark sighting at Brighton Beach on Tuesday afternoon. Photo: VicEmergency

Channel Nine reported a shark sighting on Hampton beach on Sunday, a few hours after a Nippers life saving competition.

On Monday, the St Kilda beach was closed at 8pm following a confirmed sighting of a small shark close to the beach.

With the weather forecast to remain in the high-20s for the rest of the week, authorities are warning swimmers  to be on alert.  

​Fisheries Victoria's Travis Dowling said the warm weather combined with the warm nights and people flocking to the beaches meant it wasn't unusual for sharks to be noticed in close.

Mr Dowling told Channel 10 not all species of sharks were aggressive, but things could easily change.

"You never know ... if something makes them aggressive, or if someone has got a cut on their leg," he said.

"We just don't want an unfortunate incident to occur.

"Even small sharks can take a chunk out of you, so best to leave the water, let them move on, always time for a dip later on."

A Life Saving Victoria spokeswoman said there had been an increase in shark sightings in Port Phillip Bay this summer compared to last year.

"There are always sharks in the ocean, though it is rare to see them close to shore," she said.

"The best way to keep safe in the water is to choose a patrolled beach and swim between the red and yellow flags, and to obey beach closures advised by lifesavers, lifeguards or local authorities," the spokeswoman said.

"The public can keep informed of the shark sightings on the VicEmergency website or app," she said.