A dead rat has been found in a trap on Great Mercury Island, just over two years after the island was cleared of cats, rats, mice in a $1.5 million operation.

The rat was found in Titree Bay at the southern end of the island two weeks ago.

It was killed in one of the 300 sentinel traps helping to protect the island from rats reinvading says, DOC island biosecurity advisor Pete Corson.

The rat is thought to have jumped off a passing boat.

The discovery sparked an immediate incursion response to establish it was the only individual rat that has made it ashore.

'Boats only need to be nearby for a stowaway rat to threaten the whole island,” says one of the island owners, Sir Michael Fay.

Rats are known to swim up to 1.5 km. The distance from the mainland to Great Mercury Island is too far away for rats to swim.

Intensive checks will continue throughout November as a part of the incursion response. Stopping pests getting onto boats is by far the best way of protecting islands, says Pete.

If you see anything suspicious on or around islands, please contact the Department of Conservation hotline, 0800 DOCHOT.

The rat on Great Mercury Island highlights the importance of boat users remaining vigilant with what is on their boats, as the protection of the islands is everyone's responsibility.

'The public are a huge asset for Great Mercury,” says Sir Michael.

'The island relies entirely on the visiting public to remain pest free.”

An incursion response was immediately initiated to establish it was the only individual rat that has made it ashore.

The eradication of rats and cats from 1872 ha Great Mercury Island took four years to complete, two years of planning and two operational years.

It was the last of the islands in the Mercury group to be made pest free, and the only one accessible to the public.

The recovery of wildlife on the island has been faster than expected, says Peter Cawson.

The other six Mercury Islands are incredibly sensitive Nature Reserves and Wildlife Sanctuaries with tens of thousands of seabirds and home to the impressive Mercury Islands tusked weta which was perilously close to extinction before being released on further pest free islands.

Most islands on the Eastern Coromandel are pest free including Cuvier Island, the Ohinau Islands, Castle Island and the Aldermen Islands.