A monster rat nearly 40cm long has been caught in a trap near Rotorua.

The beast was found by Rotorua Canopy Tours conservation lead and guide Scott Davis while guiding a group around the area near Mamaku Village last week.

And to say he was stunned was an understatement.

"It was definitely the biggest one I've ever seen and the large head indicated it was a young rat, too," Davis said.

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The rat measured nearly 40cm.
The rat measured nearly 40cm.

"When I saw it in the trap, I thought it was a possum."

The monster rat was found in a newly protected 10ha area near Mamaku village west of Rotorua where Canopy Tours started trapping in May as part of TRENZ, New Zealand's largest tourism conference.

About 100 traps were laid in the new section of forest.

Last week, tourism representatives returned to the forest to check the trap network when the monster rat was discovered.

The trapping network went from catching 100 rats per week to nearly seven per week in the past few months.

The rat measured nearly 40cm.
The rat measured nearly 40cm.

Canopy Tours general manager Paul Button said rubbish dumping combined with the "mega mast" could be what's responsible for the large rats being found this year.

A mast year is when trees and other plants produce significantly high volumes of seeds, and a mega mast is when even more seeds are produced.

"We're pretty sure that's what's bulking these rats up, along with the rubbish dumping in urban centres," Button said.

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"They're actually massive at the moment."

The Department of Conservation said this year had been the biggest beech mast event in nearly 50 years which has led to record-breaking levels of seed for rats to feed on in forests.

The rat measured nearly 40cm.
The rat measured nearly 40cm.

Winter meant the seeds ran out and rats would turn to native birds as a source of food.

"I personally doubt that the urban monster rats we're seeing are bulking up due to the mega mast," Button said.

"Really, it's up to us humans to ensure our waste and rubbish is managed correctly to avoid creating a feeding frenzy for rats in urban centres."

Rotorua Canopy Tours began a large-scale trapping programme in 2013 and used automatic and manual possum and rat traps.

"We've managed to almost entirely eradicate pests in the Dansey Rd Scenic Reserve," Button said.

He said finding monster rats highlighted the necessity for this "crucial work" in New Zealand's native forests.

The programme has been successful with 280ha of ancient native forest restored since the programme launched with 652 traps in total laid across the forest.

Button said there was now more wildlife and a healthier forest as a result of the conservation efforts in Dansey Rd Scenic Reserve.

"We're so excited to see the same in the Mamaku forest area."