Melbourne anglers landed the southern bluefin tuna on Thursday in 45 metres of water, about four kilometres off Portland, while fishing with Reel Time Charters.

Portland fisherman Bob McPherson weighed the fish with the charter boat's owner Matt Cini and found it to be a whopping 168 kilograms.

"We have weighed 1000 fish over the last 16 years, we thought it would be about 146 kilos, we didn't think it would be that heavy. It was a fairly fat fish," Mr McPherson said.

"I didn't expect it to go that big. Very few people have ever seen them that size."

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He said the fish could well have been heavier, but it was bled out before it was weighed.

Mr McPherson said he believed Australia's largest recorded tuna was a 174-kilogram catch caught about three years ago in waters off South Australia, west of Port Macdonnell.

"In terms of southern bluefin tuna it's probably way up there in the clouds. It's one of the biggest ones ever caught," Mr McPherson said.

"The experts reckon, and they can't prove it, they might grow to 180 to 200 kilos."

He said the catch could fetch up to $500,000 on the Japanese market, but amateur fisherman weren't allowed to sell the fish without a commercial licence.

Mr McPherson said the fishermen and his mates would be eating tuna for weeks to come.

He said local fishers wanting to land a big catch needn't look further than their own backyard.

"All these fish are in the same zone, you don't have to be in Portland to catch a fish of this calibre, Warrnambool has these fish," Mr McPherson said.

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