The myrtle rust plant disease has been found in the Wellington region for the first time.
The fungal disease - which attacks trees like pōhutukawa and rata - was first discovered in Northland in May, and has since been confirmed in Taranaki, Bay of Plenty, Waikato and Auckland.
The Ministry for Primary Industries said the disease had been found on three Ramarama plants in Lower Hutt, which were heavily infected.
The Ministry said the find was disappointing because it was now significantly further south.
More than 2000 potential sightings of myrtle rust have been reported to the Ministry of Primary Industries since May this year.
MPI said the disease thrives in a humid and warm environment, so while it was unlikely to affect plants across the whole country, more cases are expected over summer.
Meanwhile, the national plant growers' association expects myrtle rust to spread even more aggressively over the next few months.
New Zealand Plant Producers' chief executive Matthew Dolan said it was widespread now and it was time to develop a plan.
"It's likely that the spread of the disease is going to start to accelerate as temperatures warm up and MPI are aware of this. It's a case of moving into a long-term management plan for this disease - like many other plant diseases... that we manage.
"We've always suspected that myrtle rust has been widespread throughout New Zealand ... it's just a case of it exhibiting itself and the disease growing. It's possible that it's already in the South Island, it's just that it hasn't exhibited itself because it takes warm temperatures and climatic conditions."
Mr Dolan said it would be a nervous summer for South Island gardeners as they waited to see how far the fungal disease would carry.
MPI myrtle rust response co-ordinator Catherine Duthie said the latest discovery was unexpected.
"We had thought that maybe there was an off chance we could've had this disease contained to the Taranaki and Te Puke regions. Wellington is a bit of a surprise and very disappointing. It's a lot cooler in Wellington than myrtle rust really likes, but we've had a lot of warm weather recently."
She said the disease's spread was a sign that myrtle rust could be here to stay - and MPI may have to switch its focus from eradication to management.
What to do if you spot what looks like myrtle rust: (source: MPI)
- Don't touch it or try to collect samples as this may increase the spread of the disease.
- Call the MPI Exotic Pest and Disease Hotline immediately on 0800 80 99 66.
- If you have a camera or phone camera, take clear photos, including the whole plant, the whole affected leaf, and a close-up of the spores or affected area of the plant.
- Do not attempt to self-treat trees and plants with fungicide; MPI says this could prevent it "making the best management decisions for the country".