Samples from a third Northland site are being tested for myrtle rust, the Ministry for Primary Industries says.

But it says there remains just one property where the disease has been confirmed.

Plants with rust-like symptoms were found at a second Kerirkeri plant nursery on Sunday and laboratory tests on Monday came back negative.

MPI director response Geoff Gwyn said the lab team is unable to guarantee that this is not a false negative result.

"The field team have resampled at the site and will send fresh samples to the lab," he said.

"As a precautionary measure, there are controls in place on the second nursery restricting the movements of people and plants out of the property."

Mr Gwyn said the lab was also testing samples from a third location where suspected myrtle rust had been found.

The property neighboured the original infected nursery.

"We're dealing with a very fluid situation here, given the ease with which this fungus can be spread on the wind, on contaminated footwear and clothing and on tools," Mr Gwyn said.

"Our staff are following stringent protocols to avoid spreading the disease."

Myrtle rust infection is a threat to various species of native and introduced plants in the myrtle family, including pohutukawa, rata, manuka, gum, bottlebrush and feijoa.

Mr Gwyn said the disease was known to have different effects on plants from country to country and it was still now known how it would behave in New Zealand conditions.

A team of 70 people from difference agencies have been on the ground in Kerikeri since the weekend.

Mr Gwyn said the main job continued to be inspecting the area in a 500m radius from the initial affected location.