A needle in a haystack - or a single rat on a 220-hectare island.

DOC has 6 staff and two dogs on Tiritiri Matangi in the Hauraki Gulf, setting and checking hundreds of traps after finding evidence a rat has made its way to the pest-free wildlife sanctuary.

There are also 10 scientists and rat behaviour experts working from the mainland.

DOC Auckland operations manager Keith Gell said they could find something tomorrow, it could be next week, they've just got to keep going.

He said it's likely the rat came over to the island on a boat or in someone's backpack, but it is possible it floated over to the island on a piece of driftwood after last week's storm.

"We're on the doorstep of a million and a half people so it's a constant challenge in that respect, it doesn't take much for a rat to get out here, whether it be on a bit of driftwood or on a boat."

He said they have narrowed down the likely location of the rat on Tiritiri Matangi Island to a smaller area of 600,000 square metres.

Mr Gell said if unfound, the rat could wreak havoc on the island.

"We've got a lot of endangered species on this island, particularly with birds, invertebrates. They eat weta, they eat birds eggs, they eat young chicks and a lot of it is how hungry they are."

The team are worried the rat could be pregnant, exponentially increasing the problem if it gives birth, and making it hungrier in the meantime.