A monster fish nearly 2 metres long and about 150 kilograms has been found by a couple taking a stroll along a beach at Bundaberg in southern Queensland.

John and Riley Lindholm came across the animal while walking down Moore Park Beach on Tuesday afternoon.

The couple were visiting from Hastings in Victoria and planning to make a move to the region.

Mr Lindholm says the fish was about 1.7 metres long. ( Supplied: Riley Lindholm )

Mr Lindholm has spent his life around fish working as a charter skipper, but said he had no idea what it was.

"I've seen a lot of fish, and a lot of big fish, but I've never seen anything like it," Mr Lindholm said.

"I thought it might have been a groper, but looking at the head shape it still may be a groper, but it just doesn't seem to fit with what other people up here have told me."

'Took my breath away'

Mr Lindholm estimated the fish was 1.5m to 1.7m long.

"It was almost as tall as me — Riley's disappointed she didn't get me to lay down beside it so she could get a better photo," Mr Lindholm said.

"I'd estimate its weight in the range of 150 [kilograms] to 170 [kilograms] — it was a big, big fish."

He said it was one of the most unusual things he had come across.

"I've seen whales wash up on the beach but the size of this and the kind of fish it was, it took my breath away," he said.

Mr Lindholm said they posted photos on a local forum on social media to see if anyone could identify the creature.

"[There's] a fair bit of speculation — it could have been a cod, could have been a groper, and one guy I think might have nailed it when he said it was what they call a tripletail," he said.

"I've seen a lot of fish and its pectoral fin right down near the tail looked a bit odd, sort of looked like it had a joint there, and if it had the same on the other side that could be exactly what it is."

He said it had not appeared to have any physical damage to suggest it had been hit by a trawler or injured in another similar way.

"I just think it may have reached the end of its lifespan and basically expired," he said.

He said they went back the next day and the fish was gone.

The fish appears to be a Queensland groper, authorities say. ( Supplied: Riley Lindholm )

'Definitive identification difficult,' authorities say

A spokesperson for Queensland Boating and Fisheries Patrol (QBFP) said it had consulted with experts at the Queensland Museum to identify the species of fish.

Although, the "condition of the fish made a definitive identification difficult", it appeared to be a Queensland groper, the QBFP said.

"How the fish came to be washed up on the beach and its cause of death also could not be determined," the spokesperson said.

"The Queensland groper is a no-take species — in Queensland, catching and possessing this fish is prohibited.

"If accidentally caught, protected species should not be removed from the water — they should be immediately and carefully returned to the water."

The spokesperson said if anyone suspected illegal fishing activity, it should be reported to Fishwatch by phoning the 24-hour toll-free hotline on 1800 017 116.