Primary Industries Minister Nathan Guy said authorities were notified on Tuesday evening of a nursery in Kerikeri where pohutakawa seedlings had suspected myrtle rust, and laboratory testing has since confirmed the disease.
"[The Ministry of Primary Industries] has moved quickly and initiated a Restricted Place notice to restrict the movement of any plants and people at the site, and is treating nursery stock with fungicide spray as a precaution," Guy said.
Work was also underway to trace any stock that had left the nursery and all other nurseries in Kerikeri were being inspected today.
The disease is prevalent in eastern Australia and Tasmania, and was discovered on Raoul Island in late March this year.
Myrtle rust spores are microscopic and can easily spread across large distances by wind.
Officials believe that wind was the likely pathway of incursion into Raoul Island, and it was likely that wind has carried spores to mainland New Zealand from Australia, Guy said.
Conservation Minister Maggie Barry acknowledged the incursion could have serious consequences for some native species.
"Myrtle rust generally attacks soft new leaf growth, and severe infestations can kill affected plants," Barry said.
"This could include native species like the pohutakawa and the rata."
In Australia, the fungus has had different levels of impact on myrtle species, with some more seriously affected than others.
There, it had caused the extinction of several treasured plant species of significance to Aboriginal Australians.
"Myrtle rust has long been expected to arrive in New Zealand, and since the Australian outbreak began in 2010, the Government has worked on a range of measures to help manage and adapt to the fungus in the long term if necessary," Barry said.
"This includes accelerating work already underway to collect and store germplasm from affected species, searching for signs of resistant myrtle strains which could be incorporated into a breeding programme and monitoring at 800 locations across the country."
The Department of Conservation would also be conducting inspections of myrtle species on public conservation land in Northland for any early signs of the fungus.
There was no known method of controlling the disease in the wild, apart from application of fungicide in very small areas as a last resort.
Even if eradication is achieved, there was an ongoing risk of reinfection from Australia.
"It's very disappointing that this fungal disease has appeared in New Zealand," Guy said.
"However, I want to thank the staff and owner of Kerikeri Plant Production nursery for making such a prompt notification to MPI."
Anyone believing they have seen myrtle rust on plants in New Zealand were asked to call MPI on 0800 80 99 66.
It was important for people not to touch the plants or attempt to collect samples as this would spread the disease.
Study was preparing NZ for incursion
The incursion comes as scientists have been working on how to combat a myrtle rust incursion.
Last year, the Biological Heritage National Science Challenge funded a multidisciplinary research project aimed at boosting the preparedness of New Zealand's biosecurity system for an incursion of the plant pathogen.
A major strand to the project was to make better use of surveillance data to inform incursion responses just like the one we are now facing.
The research team recognised early on the potential impacts of the pathogen, and had a network of citizen scientists and researchers ready to act.
The research project, "Maori solutions to biosecurity threats and incursions to taonga species" is led by a Maori research team from the BioProtection Research Centre at Lincoln University in collaboration with Te Turi Whakamataki, Auckland Council, Scion, Plant and Food Research, Better Border Biosecurity, the Department of Conservation, the Ministry for Primary Industries, and international collaborators in Australia, South Africa, the UK, and the US.
