The M. bovis cattle disease entered New Zealand through someone breaching biosecurity rules, Prime Minister Jacinda Ardern says.

Ardern would not give details of the incursion because of an ongoing investigation, but told RNZ there were some theories on how it got into the country.

"I'm not going to prejudice an investigation," she said.

"There's no question this has come into New Zealand by someone breaching some of our rules and regulations.

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"There's biosecurity controls in place that, if they'd been followed, we wouldn't have mycoplasma bovis in the country."

Ardern would not be drawn on whether charges would be laid because the Ministry for Primary Industries was still investigating.

"I've got to leave them to do their job and I think probably the farming community would want us to let them do that too."

The Government announced yesterday that an attempt would be made to eradicate M. bovis at a cost of $886 million over 10 years, involving the culling of more than 150,000 cattle.

Ardern said yesterday it was a tough decision.

"I empathise fully with those farmers going through the pain of losing their herds."

Mycoplasma bovis is a bacterium that causes udder infections (mastitis), abortion, pneumonia and arthritis in cattle.

It does not infect humans and presents no food safety risk.

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Ardern said yesterday that to not act would cost even more than what would be spent on trying to eradicate it, around $1.3 billion.

"We have this one shot to eradicate, and we are taking it together.

"We want New Zealand to be free of it."

Agriculture Minister Damien O'Connor today announced the appointment of a science adviser, 50 more staff for MPI and a new field headquarters.

Dr John Roche has been tasked with researching new tools for the fight against M bovis.

"Dr Roche has a PhD in ruminant nutrition from the National University of Ireland and has most recently worked as a principal scientist at DairyNZ and adjunct professor in animal science at Lincoln University," O'Connor said.

"He will provide strategic science advice across MPI and his first task will be to head up a new Mycoplasma bovis science strategic advisory group.

"There has been little international investment in science around this disease so the group will look into testing developments to detect Mycoplasma bovis in individual cows, grow understanding of the disease and identify opportunities to support the New Zealand eradication operation," O'Connor said.