Fears a rat may have made its way to an offshore predator-free kākāpō sanctuary have been allayed after DNA analysis of the suspect droppings revealed it to likely have come from kererū.

Pest detector dogs and additional traps were deployed on Whenua Hou, also known as Codfish Island, last week after a dropping believed to have come from a Norway rat was found.

The Department of Conservation's Murihiku operations manager, Tony Preston, said he was thrilled DNA results ruled out the Norway rat as the source of the dropping.

It raised alarms as the dropping was not the shape or look of that generally expected from kererū, something which diet or environment might account for, he said.

"Whatever the factors, we'd much rather be dealing with a strange kererū poo than a rat incursion," Mr Preston said.

The fears were spiked as a mottled petrel carcass was found with signs it died as a result of another animal.

A necropsy suggested it died as a result of a harrier, he said.

"Whenua Hou is one of our most important and valuable sites for biodiversity and any evidence of incursion is treated very seriously," Mr Preston said.

"Rats breed at an alarming rate - one female can reproduce up to 12 times a year, having between eight to 12 pups per litter. With these kind of timeframes, days and weeks matter - it's best to take a better safe than sorry approach."

There was already a network of traps on the island, however, plans were in place to increase the volume of these in the new year, so it was a bonus to be able to extend the network before Christmas, he said.

Pest detector dogs would also remain on the island to conduct a thorough search over the next week for additional peace of mind.