About 3500 Napier households have been warned to look out for stinging red fire ants.

Residents have been sent flyers telling them to expect possible visits by Ministry of Agriculture and Forestry (MAF) surveyors.

About 200 of the red fire ants were discovered at the Port of Napier during routine checks by MAF on February 12, triggering a biosecurity alert and eradication campaign.

The reddish-brown worker ants, which vary in size from 3-6mm and have a sting similar to a bee or wasp, were discovered in baited traps set by MAF.

MAF national invasive pest programme co-ordinator Amelia Pascoe said toxic baits appeared to have eradicated most of the colony, with the last live ant seen on Friday.

MAF officials would continue to lay traps within 500m of the site until April 2, Ms Pascoe said.

No red fire ants had been found in the traps, but a small number had been found and eradicated in a container about 700m from the site, Ms Pascoe said.

The container was thought to have been stored beside the nest before the colony was discovered.

Samples of the ants had been sent to Australia and the United States, where tests had confirmed they were a monogyne variety, meaning they belonged to a group that has just one queen ant.

This meant there was likely to be just one nest, as opposed to several, Ms Pascoe said.

"The advantage of monogyne ants is that they are unable to reproduce unless they have the queen with them," she said.

MAF officials will return to the site next summer to ensure the area is not reinfested.

"We need to find no live ants for two years to be confident the area is clear," Ms Pascoe said.

It was the second discovery of fire ants in New Zealand. In March 2001 a single nest was found at Auckland International Airport. The nest was destroyed.