Three fishermen braved possibly shark-infested waters in the Hauraki Gulf to haul up a “one-in-a-trillion” catch, a monster tuna weighing 155kg.

Cameron Ellin, Brodie Ellin and Josh Jenkins were just north of Little Barrier Island on Thursday morning hoping for kingfish when they spotted what they thought was a trapped seal.

They decided to get a closer look. When they investigated the supposed seal they were shocked to find it was a tuna squirming around, caught between some rocks.

“We thought it was just like a small-size tuna. And then when we swam up to it we were like ‘Oh, wait a minute’,” Cameron Ellin told the Herald.

Brodie Ellin sizes up a fish he found stuck in rocks off Little Barrier Island when he was fishing with his brother, Cameron Ellin, and Josh Jenkins.

Ellin said they tied a rope around its tail to pull it out, but it was harrowing as the boys feared sharks attracted to the fish’s blood may have been circling.

“We were hoping that there wasn’t going to be sharks everywhere, because the visibility was only like 2 metres in the water and it was like pitch black, it was real black water.

“But we swam in and tied that rope to it, but we weren’t too sure what to do to be fair. We were kind of torn between pulling it out or dispatching [striking and killing] the fish first and then pulling it out because obviously a big fish like that, what’s it going to do?

“Two of us managed to pull it free and then we put a spear gun into it once we got it free and that finished it off.

Ellin, his brother and their mate say it was a southern bluefin tuna, coming in at 155kg. At 2.3m it was bigger than them.

“Honestly, it would be a one in a trillion. You’re never going to find something like that ever again.”

The Herald Sun in Australia said the world’s biggest southern bluefin, weighing 178kg, was caught in 2015. The biggest bluefin was caught off Nova Scotia in Canada in 1979, weighing 678kg, according to Sport Fishing magazine.