Dwayne Aberhart poses with a monster 34 pound brown trout caught at the Ohau C Canal.

Anglers are describing it as the biggest looking fish ever pulled from the South Island's hydro canals, but a man who was there when it was caught is convinced there's even bigger monsters lurking in there.

Mates Dwayne Aberhart and Logan Ahlfeld were doing some late night fishing at the Ohau C Canal on September 10 when Aberhart snagged what the pair say was a 15.4kg (34 pound) brown trout on his line about 2am.

He fought with it for 25 minutes before hauling it to the edge of the canal, the hook pulling from its mouth just as he netted it.

The two posed for photos before putting the monster back.

"I was quite amazed at the size of it," Ahlfeld said.

Logan Ahlfeld with his 23-pound catch on the same night. Photo: Facebook

"They seem to come out at night, that's what we've found anyway. We've been for night walks and we've seen fish in these canals that are probably more than 40 pounds."

Ahlfeld, from Rangiora, has been fishing since he was a young boy, but said the catches that night – which included a 23 pounder that he had hauled in – rated "right at the top" of his angling experiences.

He posted a photo of Aberhart holding the 34-pound fish to the Canal Fishing - Twizel/Tekapo Facebook page on Wednesday and on Friday morning it had more that 1600 likes and had been shared 555 times.

"I didn't think that many people would respond to it like that," Ahlfeld said.

"We'll definitely be going back again."

The pair had an electronic weighing device on their net but Ahlfeld said it could easily have weighed more than 34-pounds at they had struggled to lift the net off the ground to get the tension required.

South Canterbury-based fishing columnist Peter Shutt said it was the biggest fish he had ever seen pulled out of the region's freshwater canals.

"That's just a very, very big fish. I'm amazed they were able to lift it [for photos]. It looks a lot bigger than 34 pounds.

"I've seen fish of a similar size but nothing that heavy and blown up, usually the trout in there are very sleek."

After posing for photos the two mates put the trout back in the canal. Photo: Facebook

Shutt said it looked like a female and it was almost certainly a cannibal.

"It's eating other fish in the canal, no doubt about it."

In March 2013 fisherman Otwin Kandolf hooked a 20.5-kilogram (45 pound) brown trout in the Ohau B canal. It took nearly two weeks for an official weigh-in by which time the fish weighed 19.10kg.

*An earlier version of this story said Ahlfeld had caught the 34 pound fish.