A group of young fishermen has had a terrifying brush with a rogue Great White shark when the five-metre predator attacked their small boat in Victorian waters.

Ben Donegan, his brother Andrew and friend Joel Ryan were tuna fishing at Cape Nelson, off Portland, near the Victorian and South Australian border, on Saturday when the huge beast approached their vessel.

The trio said the shark began circling the boat - and then went on the attack.

They managed to film the amazing encounter using an underwater camera.

"This thing just cruised in," Ben explained. "There was absolute silence and then this massive shape, pretty much the length of the boat, went under the boat and then disappeared, and popped out on the other side.

"You think the worst...what if it tries to grab the boat and pull you under?

"Our hearts were beating out of our chests."

Shark attack caught on camera. Photo: Supplied

The men said the shark initially seemed quite calm in temperament as it gracefully stalked the boat, but after about 20 minutes it became aggressive.

"It was quite calm then it started to nudge the boat, bumping us and biting things," Ben said.

The men decided to escape when the shark closed in - then locked its jaws onto the motor.

Andrew added: "It started to get a bit angry and that's when we decided to get out of there."

Shark attack caught on camera. Photo: Supplied

The men believe the shark had been attracted to electrical pulses coming from anodes on the engine, designed to stop it rusting.

"We spend a lot of time in the water so to see something like this is a once-in-a-lifetime moment," Ben said.

"It was a very humbling experience to see something so big, so close," Andrew added.

Shark attack caught on camera. Photo: Supplied