Taranaki's first case of Mycoplasma bovis has been confirmed.
The Ministry for Primary Industries said the herd on a Taranaki property had tested positive for the disease.
A Restricted Place notice has been placed on the farm.
MPI Mycoplasma bovis director Geoff Gwyn said the M. bovis response team had worked extremely hard to establish whether the property was at risk of being infected and had established the risk was high, as was the risk of spreading it to other properties.
Gwyn said it was a sensitive situation and extremely trying for the farmers involved.
"For privacy reasons we cannot go into specifics of the property involved, however I can tell you that a number of animals will be culled.
"We have not taken this decision lightly and it is necessary for the response to eradicate M. bovis from New Zealand."
He said MPI was working with the farmers and their support networks to help them through the process.
"While every decision is made with the aim of eradicating the disease from New Zealand, we are well aware that the effect of this eradication response has on individual farmers. We are wrapping as much welfare support around the owners of the property as possible and are working with them and their support networks to ensure they are well supported through this process."
Gwyn said the bacteria was easily spread through animal contacts or drinking infected milk, so any cattle which had mixed with infected animals were considered to be at risk of infection even if they did not show clinical signs.
"This is a fundamental tenet of the response and strongly supported by our external Technical Advisory Group. This means, if there is any confirmed infection in the movement group, we need to assume all animals in the group are positive."
All infected farms, and farms suspected of having Mycoplasma bovis have a Restricted Place Notice in place.
This prohibits all unauthorised movements of farm stock and other risk goods on to and off the property. This minimises the chance of the disease spreading from the property.
Any movement of cattle requires a permit from MPI.
Transport vehicles must follow a cleaning and disinfection process when they leave a restricted place.