The number of locations infected with myrtle rust has risen to eight and more are expected to be found.
On Saturday, MPI said three new locations have tested positive: two residential gardens in Waitara and a Taranaki Regional Council-run plant depot which supplies plants to farmers for riparian planting.
Myrtle rust attacks native trees, such as pohutukawa and rata, and could cause serious damage to manuka trees, used in honey production.
There is no known method for controlling it in the wild, other than applying fungicide in very small areas.
The council plant depot has been closed and is being treated.
Four other similar plant depots in Taranaki are being investigated and farmers won't be able to collect trees from them next week.
The eight infected properties are a nursery and adjoining residential property in Kerikeri, two plant nurseries in Waitara, two Taranaki residential gardens, a garden centre in New Plymouth, and the regional council plant depot.
MPI says it expects to continue to find new locations of infection as it was likely the fungal spores entered New Zealand from Australia during a major wind event.
There are restrictions on the movement of plants or other risky materials off the infected sites.
They are being treated with fungicide, plants at risk are being destroyed, and surrounding properties are being checked.