Myrtle rust has been confirmed in two more properties in Taranaki.
The Ministry for Primary Industries (MPI) said a plant nursery in Waitara and a garden centre in downtown New Plymouth have both been infected.
There are now five properties confirmed contaminated with myrtle rust - two in Kerikeri, two in Waitara and one in New Plymouth.
The fungal infection has been killing large numbers of native trees in Australia since 2010.
It threatens New Zealand natives including pōhutukawa, rātā, mānuka and kānuka, as well as introduced species such as feijoas, guavas, gums, bottlebrush and other members of the myrtaceae family.
MPI said it was not confident it could eradicate myrtle rust after the two new cases were confirmed.
David Yard from the ministry said scientists think myrtle rust has been slowly growing in the country after spores were blown over from Australia.
"Our scientists are tell us [the plants] have been infected between three weeks and two months already before they show symptoms," he said.
"So in many cases other premises or other trees out in the wild might be infected and we're just waiting for people to report them for us to become aware of them."
The ministry said it expected to find more cases of the disease because it was most likely being spread by the wind.
It said infected properties were being treated with fungicide and plants with the rust were being destroyed.
MPI has previously said that the continued spread of the infection was a sign that we may need to learn to live with it.