Laboratory tests have confirmed that the deadly plant fungus myrtle rust has spread beyond the Kerikeri nursery where it was first found, and government officials have conceded that it might be impossible to stop it spreading further.

Confirmation of the spread came from samples taken from a tree on a residential property close to Kerikeri Plant Production, where the fungus was discovered by the nursery owners last week.

The Ministry for Primary Industries (MPI) has placed conditions on the property to prevent the spread of the rust, and the affected tree is being sprayed with fungicide.

Signs of the fungus have been found on another property neighbouring the original nursery but have yet to be confirmed.

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Re-testing of samples from a second, as yet un-named nursery was continuing on Tuesday. Plants from the second nursery, which had bought seedlings from Kerikeri Plant Production, appeared to be infected but initial lab tests came back negative.

Geoff Gwyn, who is heading MPI's response, said there were now 100 staff on the ground in Kerikeri from MPI, DOC, the Northland Regional Council and AsureQuality. Checks on all properties within 500 metres of the two confirmed finds had been completed.

MPI was still committed to preventing the spread of myrtle rust, but "We have to be realistic that this is a disease that spreads by microscopic spores that can be carried by the wind and on people, vehicles and equipment. Containing it may not be possible."

Myrtle rust had not been eradicated successfully anywhere in the world, he added.

Earlier Primary Industries Minister Nathan Guy conceded that keeping myrtle rust out of New Zealand long-term appeared to be a losing battle, given expert opinion that up to 20 weather events a year were capable of blowing the spores over from Australia.

However, if the current spread could be stopped it would buy time for scientists and industry groups to hone plans for dealing with the disease.

"We need to treat this very seriously ... We have a small chance of eradication, but we are always going to be at risk because of the weather patterns, the strong westerly winds and weather events blowing all sorts of things on to Northland," he said.

The disease threatens native species such as pohutukawa, manuka and rata, as well as feijoa, bottle brush and eucalypts. It has been present since 2010 in Australia, where it has devastated some native plant species. Its effects vary from plant to plant, and it is not yet clear what it will do to New Zealand members of the myrtle family.