Three hours later, the two men returned to shore with a passenger that was beyond their wildest dreams.

Saltmarsh and Tuthill landed themselves a prized bluefin tuna weighing in at a whopping 119.3 kilograms.

Saltmarsh said to catch something so big in local waters was unheard of.

"It's a dream fish," he said. "We've tried to catch something like this for a long time at places like Eaglehawk Neck and St Helens, but to catch it 5-10 kilometres from home is unbelievable.

"It's very rare to catch a normal-sized bluefin tuna off the North-West Coast or even the top end of Tasmania - they're pretty standard along the East Coast and you get a few on the West Coast.

"We've caught a lot in the 15-25kg range, but this is by far the biggest one I've been on board for."

The pair spent 90 minutes trying to reel the fish in, swapping shifts midway through as fatigue set in.

Originally they thought they may have caught something else, but quickly realised it was a bluefin tuna.

"There was a little bit of hesitation when we first hooked it because we thought we may have landed a dolphin," Saltmarsh said.

"But when it stayed down for 4-5 minutes we ruled that out.

"We knew it was big because it took 300 metes of line in less than five minutes and that was 80-pound braid - we had smoke coming off the reel."

So what do you do with close to 120kg of tuna?

"We're very stoked to have this much tuna on our hands and we're filleting him up right now," Saltmarsh said.

"There will be some big feeds of tuna coming up for family and friends."