Rodent-killer Pai is New Zealand's newest hero in the fight against unwelcome pests after he caught a rat destined for an island wildlife reserve.

The trained Department of Conservation border terrier cross yesterday detected and killed a rat that was on a van about to be ferried to Motuihe Island, a pest-free reserve in the Hauraki Gulf.

DoC ranger Fin Buchanan hailed Pai as a hero for his efforts because if the rat was not detected it could have wiped out of a number of endangered species.

"I must admit I'm very proud of Pai. I feel like the father of an All Black who has just scored a try in a test match," Mr Buchanan said.

Motuihe Island has been free of rats and other animal pests since 1996.

Mr Buchanan said if the rat made it to Motuihe, endangered species including kakariki, bellbirds, saddleback and shore skinks would have been at risk. "It's hard to overstate the significance of catching and killing this rat before it got to Motuihe Island.

"There was also the risk that if a rat population had developed on Motuihe, rats could have swum to neighbouring pest free islands such as Browns and Rakino which are also safe havens for endangered wildlife," he said.

The ranger said Pai detected the rat around midday when he carried out a routine check of cargo being loaded onto the ferry at Half Moon Bay in Howick.

"Pai began straining on his lead as soon as he began checking the van. He knew there was a rodent somewhere on the vehicle.

"We got the van off the ferry. The rat appeared to be in the engine area. I got into the cab of the van and began banging on the area above the engine. Pai was under the engine ... The rat dropped out of the engine and tried to escape but Pai caught and killed it."