Peter Galea and his friend Joseph McKinnon, both of Portland, were fishing near the Water Tower at Anderson Point on Thursday when the shark began chewing the boat's motor about 1.30pm.

It came after shark warnings were issued where two whale carcasses had washed up in Portland, as reported by the Victorian Fisheries Authority through the VicEmergency app.

Mr Galea estimated the shark he sighted was about four or five metres long and at some point went under the boat.

"He went under, was big and just lifted the boat a bit," Mr Galea said.

"It was mayhem in the boat because we had rods out and wanted to bring the rods in and get the berley buckets up.

"It's not the first time I've seen a shark, but the first time a great white has been at the back of the boat.

"My mate and I were fishing out there for kingfish and all of a sudden a great white came out and started having a chew on the motor at the back of the boat.

"The young bloke (Joseph) got scared as it's the first time he's seen something like that."

Mr Galea said the shark "hung around" for about an-hour-and-a-half.

"There were another 50 or 60 boats out there but for some reason it came up to our boat," he said.

"Some other boats came behind us to take videos."

Mr Galea said each time he started the motor the shark left the pair alone for about 10 minutes then would return for another bite.

"For some reason he liked our boat," Mr Galea said.

"It was frightening and scary all at the same time.

"He wouldn't go away, he just hung around.

"Me packing up and going home is what got him to go."

IN OTHER NEWS:

Mr Galea has been fishing since he was in his early teenage years, but said the incident would not deter him from continuing to fish.

"I'm going to go out next Monday or Tuesday to get my kingfish again," Mr Galea said.

"You have to expect it sooner or later because you're in water.

"It's like when you go shooting, you've got to expect to see a snake."

The VFA said any shark more than two metres in length was considered dangerous.

"There is an increase in shark activity due to two whale carcasses in the area," a VicEmergency update said on Friday morning.

"A whale carcass can attract sharks and mean they are closer to the shore than normal.

"Avoid the area. Stay informed and do not enter the water at closed beaches.

"While it is not uncommon for sharks to be present off the Victorian coast, you should exercise additional caution in the area."

The VFA is monitoring the situation and providing advice to relevant authorities.

They said shark sightings should be reported to triple-0, or lifesavers immediately if you are at a patrolled beach.