Two young Tasmanian boys believe they have broken world records for the biggest southern bluefin tuna caught by junior fishers.

The boys were fishing with their dad at Hippolyte Rocks off the Tasman Peninsular in south-east Tasmania on Friday.

Sam Nichols, 12, is hoping to break the junior world record with his 112-kilogram catch, which he made on a 24-kilogram line.

Toby Nichols, 10, could break the "small fry" (under-11) world record with his 119-kilogram fish caught on a 37-kilogram line.

Their dad Stuart said it was a mammoth effort from both boys.

"Two hours for the first one, which turned out to be 112 kilos, second one was three-and-a-half hours - 119 kilos," he said.

"One was my 10-year-old son, one my 12-year-old son, so pretty awesome effort for young boys to catch fish like that.

"They were exhausted, they were absolutely exhausted — I didn't think they'd be able to do it on their own."

Mr Nichols said Sam only had to beat 90kg to break the world record and Toby had to top just 40kg.

"We knew pretty much straight away they were world-record size for their age, so we're pretty excited," he said.

He said the record first needed to be ratified by the local tuna club.

"It then goes to the state body and then it goes to the Australian body," Mr Nichols said.

"Samples of the line is sent, the lures are sent, all measurements have been taken on the rods and reels and line, and all the things involved in the capture."