After 30 minutes battling a river monster in Ōhau Canal, Aussie angler Paul Rahman decided to take matters into his own hands - literally.

It was after midnight. The sun had long hidden behind the Southern Alps and the chill of autumn hung in the air. But Rahman did not hesitate as he dived into the canal in South Canterbury to drag out the massive brown trout on the end of his line.

Cold, wet, and victorious, he had no idea he had just made a world record catch.

The trout weighed 17.75kg and has been recognised by the International Game Fish Association (IGFA) as the heaviest brown trout caught on a 4kg line.

"I knew when it hooked the trout it was big," Rahman said, of his 15 April catch.

"Had to just keep running and pulling the line, because you're fishing such a light line on such a big fish you've got no control out of it. Even the last ten minutes, when the fish was completed buggered, it was just hard to turn in the current. They do jump a lot as well at night."

As he struggled to bring it ashore, he called for his friend to help.

"I had my friend with me and I said to him it's a big fish, when we had it closer into the bank he said 'It's not that big I don't know what took you so long to bring it in'... I said to him to jump in the water and nab it. But he wouldn't jump in so I gave him my rod and virtually dived on it and got it out," he said.

Rahman, who lived in Melbourne, had travelled regularly to Twizel to fish for more than a decade.

He was first drawn to Ōhau Canal by rumours of large trout swimming there.

"We do a lot of night fishing for the big brown trout, so we'll find fish in the water [during the day] and then come back later on... Sometimes it might take the first cast and other times it might take hours. It's very effective but can be very frustrating," he said.

The canal could be tough to catch fish in, Rahman said.

"Most of our fishing is between 8pm and 4am. All those big browns come out on the edges and they start feeding."

He recorded the weight of the massive fish on his IGFA-certified scale before releasing it.

"I don't really eat big trout and I've already got a couple of big fish on the wall mounted," he said.

It took months before the record was made official, Rahman said.

Fish and Game Central South Island chief executive Steve McKnight said it was great to hear a record brown trout had come out of Ōhau Canal.

"It's really cool to see another record come out of the canals. The current all-tackle world record brown trout was caught in the Ōhau Canal as well in 2020, and that weighed in at about 20.1 kg, so another big one," McKnight said.

He said the Ōhau Canal is part of the Mackenzie Basin hydro canal system and it is recognised as a world-class trophy trout fishery.

"There's rumours of a 50 pounder up there that's been caught. No one has recorded or verified it, we anticipate that there'll be a 50 pound fish that will be caught and made an official record at some stage."

McKnight encourages people to get out there and keep searching for him.