The fungal disease threatening iconic native trees and the manuka honey industry was most likely blown over from Australia, Primary Industries Minister Nathan Guy says.
The discovery of myrtle rust at a Kerikeri plant nursery last week has sparked a major biosecurity operation with more than 100 Ministry for Primary Industries (MPI) workers and contractors on the ground, aided by Department of Conservation rangers and staff from AssureQuality, a food safety and biosecurity firm.
Over the weekend they continued checking all properties within 500m of the infested nursery and handed out leaflets at Kerikeri supermarkets and markets.
MPI is also advertising in newspapers and radio and carrying out letterbox drops while DoC rangers are checking vulnerable stands of pohutukawa.
The nursery where the disease was found on Tuesday evening, Kerikeri Plant Production, remains on lockdown.
Mr Guy and Conservation Minister Maggie Barry visited the nursery on Friday and said the owners "did all the right things" by detecting the disease early and notifying MPI immediately.
MPI staff inspected the plants, took samples and confirmed the nursery owner's fears within 24 hours.
Mr Guy said the fungus had most likely blown over from Australia where the disease became established in 2010. The spores could be carried thousands of kilometres by the wind.
Mrs Barry said the disease could also be spread by birds, insects or people, if they touched the fungus or got spores on their clothing.
She urged people not to touch anything that looked suspicious but to take a photo and call the MPI hotline, 0800 80 99 66.
As well as natives such as pohutukawa and manuka, the disease could strike common trees and garden plants such as lilly pilly, feijoa, gum trees and bottle brushes.
The signs could take three to five weeks to emerge so she urged people to keep checking their plants.
It started as a purple patterning on the leaf that turned into a very lurid, bright yellow spore.
Geoff Gwyn, who is heading MPI's response, said on average 20 wind events a year were capable of carrying spores from Australia to Northland.
If smoke from Sydney bush fires or Australian insects, such as the guava moth, could reach New Zealand, it was no problem for spores.
The disease is already having an effect on businesses and re-planting programmes.
Nurseryman Eddie Gompelman of Waipapa voluntarily stopped his sales of manuka and pohutukawa seedlings at the Old Packhouse Market on Saturday, so his income was down by about two-thirds.
"It's a concern, especially if it takes hold, from a business point of view but also environmentally," he said.
Mr Gompelman also helps with re-planting projects along Kerikeri's Wairoa Stream and on the islands of the Bay of Islands.
A major planting exercise involving 100 volunteers and 2000 trees due to take place on Urupukapuka Island on Sunday was cancelled.
The seedlings are already on the island and not expected to survive if they are not planted soon.
The disease was found on Raoul Island, about 1100km north of Cape Reinga, earlier this year but last week's discovery was the first on mainland New Zealand.