Lab tests for myrtle rust from a suspected infection at a second Kerikeri nursery have come back negative - but now plants with rust-like symptoms have been found in a garden next to the original nursery.

Last week's discovery at a Kerikeri plant nursery of myrtle rust, a fungus with potentially devastating effects on trees such as pohutukawa, manuka, bottlebrush and feijoa, sparked a major biosecurity operation by the Ministry for Primary Industries (MPI), the Department of Conservation (DoC) and other agencies.

Kerikeri Plant Production, where the fungus was first found, is under lockdown and MPI staff are searching all properties within 500m. They have also been tracing the nursery's sales over the past three months.

Geoff Gwyn, who is heading MPI's response, said customer tracing led staff to a second, un-named Kerikeri plant nursery where plants appeared to be infected with myrtle rust.

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However, lab tests completed yesterday came back negative for the disease.

A "false negative" result could not be ruled out so field staff had taken fresh samples from the site for testing. As a precaution the second nursery had also been place in lockdown.

Yesterday lab tests were also being carried out on samples from a third location, a property next to the original infected nursery, for suspected myrtle rust.

"We're dealing with a very fluid situation," Mr Gwyn said.

Staff involved in the operation were following stringent protocols to avoid spreading the disease given how easily it could be spread by wind, contaminated footwear, clothing and tools.

As of yesterday 70 people from MPI, DoC, AsureQuality and the Northland Regional Council were on the ground in Kerikeri with numbers increasing daily.

The fungus is thought to have arrived on the wind from Australia, which has had the plant disease since 2010. There it has devastated some native plant species while others, such as mature gum trees, have show signs of the disease without being seriously affected.

Mr Gwyn said the fungus had different effects on plants from country to country and location to location.

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"We still don't know how it will behave in New Zealand conditions," he said.

■ If you think you've seen signs of myrtle rust, don't touch it or the plant. Take a photo and call MPI on 0800 80 99 66.