This is thought to be the highest concentration of rats in the world.

The rats are blamed for the disappearance of several wildlife species unique to the island, including a flightless teal duck and another wading bird.

The conservation experts placed the poisoned bait around the island in the southern winter, when the rats are not breeding and food supplies are scarce.

"The rats have removed most of the bait from the surface of the ground and cached it in their burrows," the New Zealand Department of Conversation said in a statement.

"Radio transmitters were attached to several live rats at the start of the operation, and all have since been found dead."

The rats are thought to have come ashore on the island from 19th Century whaling and sealing ships.

Each female Norway rat can produce up to seven litters with a dozen babies each year, and the population has grown prodigiously on the uninhabited island.

Radio transmitters were attached to several live rats at the start of the operation and all have since been found dead

New Zealand Department of Conservation

In May, the programme to eradicate the rats on Campbell Island started badly when a tanker carrying 18 tonnes of poison crashed near a whale breeding site.

No environmental damage was reported after the crash.

Now, the authorities report that all the poisoned bait has been distributed ahead of schedule.

The Department of Conservation says it will be two years before they know if the rat killing operation has been successful.