Eighteen farms are now on the suspect list for a new potato virus, Biosecurity New Zealand says, and testing has been widened to the North Island.
The Potato Mop-Top Virus, or PM-TV, affected potatoes used to make chips. It was found for the first time in New Zealand on two properties in Canterbury last month.
But the latest industry stakeholder update showed that as well as the two confirmed cases there were a now further 18 suspected cases, also in the Canterbury region.
The virus' response controller, David Yard of Biosecurity New Zealand, said they were testing seed potatoes, plus samples from manufacturers and growers in the North Island.
He said the suggestion at the moment was that there were not a lot of "highly symptomatic potatoes" - such as those with large purple streaks across them.
"That's not to say we're being dismissive of the disease - our focus at the moment is on finding out how far this disease has spread, and on working with industry to help manage the disease."
Mr Yard said the disease did not present a food safety risk; it was purely a production impact.
He said the discovery so far had come exclusively from processing plants.
"But clearly, if the processors have potatoes with symptoms, they have come from infected farms, and out job is to trace back."
Mr Yard said it was akin to searching for a needle in a potato stack.
"We have two confirmed farms but 18 suspect farms - that doesn't mean we suspect all 18 farms where these potatoes may have come from are all suspect. They're only suspect because the way the processing industry works - there's a huge mixing of potatoes at the plant."
He said they could find an infected potato but the industry was only able to narrow it down to one of three or four farms.
Mr Yard said the opinion of global experts on the disease was that eradication was not feasible.
"Last week, the Ministry for Primary Industries convened an international technical advisory group, where we invited international experts from around the globe," he said.
"Their opinion was that eradication is not feasible, based on the level of distribution of the host, which is right across the country, and their international experience."
Mr Yard said they were now at the stage of considering moving away from the option of eradication to working with the industry to help it manage the disease.
He said it was too early to say if there have been any production losses, and compensation to growers was only payable if legal powers were exercised.
"At the moment we've not exercised these powers and there'll be no compensation to growers."
Mr Yard said production losses would be dependent on the impact of the disease, which at this stage was largely unknown.
"From what we've seen so far it doesn't appear the impact will be as great as we feared."
The agency was also looking into how the virus entered the country.