A global "mega-colony" of Argentine ants is spreading through New Zealand, with evidence the pests are forming inter-nest alliances to dominate native species.

The ants, which have invaded all continents except Antarctica thanks to human movements, are so closely related that nests refuse to attack each other, co-operating instead to overrun native species.

Infestations of the 3mm, honey-brown pests were first reported in New Zealand in 1990. Now researchers say colonies from Northland to Christchurch come from such a small genetic pool that they treat each other as long-lost relatives.

They are part of the global colony of Argentine ants that has been reported in a study by the University of Tokyo. In Europe, the mega-colony stretches over 6000km along the Mediterranean coast; in the US, the colony stretches along the California coast for some 900km; and in Japan, a huge colony has been built on the country's west coast.

"Our research found Argentine ants from three continents were rather friendly, and not hostile towards each other," researcher Eiriki Sunamura told the Herald.

Invasive ant expert Dr Phil Lester, of Victoria University, said genetic studies showed all Argentine ants in New Zealand were likely to have come from a single Australian nest.

"We put ants from Kaitaia and ants from Wellington together and they all seemed to get along," he said.

Dr Lester said reports of giant super-colonies did not mean there was a single unbroken nest, however "you could make the argument that New Zealand is one big super-colony because they are inter-related, they don't fight, and they recognise each other".

Most ants guard their territory and would attack ants of the same species if they entered their zone.

Entomologist Richard Toft said Argentine ants had settled in cities from Northland to Christchurch after entering the country through Auckland. A large part of the city of Hastings was covered by co-operating nests of ants

Groups of individual nests might be divided by roads or other barriers, and ants from different cities would not normally meet unless they were carried by humans.

Ants were normally the best controllers of other ants, because they were naturally aggressive and liked to kill the competition, Dr Lester said.

For example, New Zealand native ants killed each other and wouldn't hesitate to kill an Argentine ant, he said. It took four or five Argentine ants to kill a single native ant. They were transient creatures, living in holes, cracks or under rocks or compost heaps, and would easily find a new nest if one was destroyed.

MAF Biosecurity's website says Argentine ants are listed by the World Conservation Union as one of the world's 100 worst invasive species. "They produce multiple queens and can form huge super-colonies that extend for thousands of kilometres," it says.