An American tourist has claimed he's found the world's biggest insect during a two-day hunt on Auckland's Little Barrier Island - but the experts aren't buying his tall tale.

Mark Moffett, a former park ranger from Colorado, found the Little Barrier giant weta up a tree and snapped it nibbling a carrot.

Moffett's photos have been published on Britain's Daily Mail news websites where he claims to have found the largest ever specimen.

"The giant weta is the largest insect in the world, and this is the biggest one ever found, she weighs the equivalent to three mice," he told the Daily Mail.

"She enjoyed the carrot so much she seemed to ignore the fact she was resting on our hands and carried on munching away.

"She would have finished the carrot very quickly, but this is an extremely endangered species and we didn't want to risk indigestion."

The nocturnal creature, known as wetapunga or "god of ugly things", is the largest sub-species of the giant weta, weighing the same as a small bird.

But Landcare Research entomologist Dr Thomas Buckley says, based on Moffett's photos, the weta's size looks about average for its species.

"The species itself is the heaviest in the world but whether that individual is the heaviest you couldn't really say.

"From the picture it's a female, but it just looks like an average sized one of that species."

The largest ever recorded weighed 71 grams - larger than the average sparrow.

Buckley says, while the species is endangered, they are fairly easy to find on the island.

"On Little Barrier they're reasonably common, you can go out at night and see them on the trunks of trees, but you won't see them on the mainland. They disappeared years ago from the mainland.

"During the day they'll be wedged away in rotten trees and hollows, and at night you'll see them out walking around and looking for mates and feeding and so on."

The wetapunga can grow up to 10 centimeters long and its leg span can reach 20 centimeters.