Myrtle rust has been found on conservation land for the first time, after a warm summer gave the wind-borne plant disease ideal conditions to spread.
Its discovery on a ramarama plant in north Taranaki bush comes as officials lifted a ban on moving plants around the region because it had failed to stop the disease.
Myrtle rust is a fungus that attacks myrtle species plants, including pohutukawa, manuka, kanuka and rata.
First found in a Kerikeri nursery last May, it has now been confirmed at more than 100 places across Taranaki, Te Puke, Waikato, Northland and, more recently, Auckland and Wellington.
But its detection, near the Kiwi Road Track in Mt Messenger, was the first time it had been found in a large tract of native bush.
The Department of Conservation was closing the area and working with Ministry for Primary Industries staff and other groups on the response.
"We're still investigating the extent of the outbreak and exploring options to contain any risk of spread," said Phil Hancock, who is leading DOC's response.
DOC had been taking an "active approach" to meet myrtle rust's threat to public conservation land, and field staff had been closely monitoring high-risk areas, mainly in the North Island and upper South Island.
The outbreak also triggered an eleventh-hour seed-banking effort around the country, with DOC directing hundreds of officials to collect seeds of more than 30 native plant species.
The recent warm weather has been ideal for myrtle rust and, with the yellow powdery spots appearing on leaves and new plant shoots, new detections have increased in some areas.
It remained unclear what damage myrtle rust could inflict upon New Zealand's myrtle species.
One assessment produced by the Government in 2011 warned large areas of the country could be affected and, at one extreme, some species could die on a "landscape scale", as had happened in other countries.
Other experts have suggested New Zealand could lose some particularly vulnerable species like Bartlett's rata, particularly as they existed in the wild and because there were only a few individuals left.
Landcare Research scientist Dr Andrea Byrom earlier told the Herald that she feared efforts to stop it may be doomed.
In Australia, the fungus has had different levels of impact on myrtle species and has caused the extinction of several treasured plant species of significance to Aboriginal Australians.
Some scientists have been critical of New Zealand's response to myrtle rust, questioning why a specific response plan had been arranged years before it arrived here, but MPI has asserted it had the necessary protocols in place.
Forest and Bird's Dr Rebecca Stirnemann called on the new Government to set up and fund a dedicated independent biosecurity agency to deal with the new threats to our native species that climate change would increasingly bring.
"It is possible to prevent and control these diseases, but MPI has repeatedly failed to do so, even when the risk is as serious as ecosystem collapse."
Meanwhile, people were being were encouraged to check myrtle species plants – particularly ramarama, pohutukawa, lilly pilly, manuka and rata.
If they believed they'd seen the disease, they shouldn't touch it, but take pictures, note the location and call the MPI biosecurity hotline immediately on 0800 80 99 66.