Four more Canterbury farms have been confirmed as being infected with Mycoplasma bovis.

The infected properties (IP) are two dry stock farms and two dairy farms. All four have received animals from already-known infected properties.

The Ministry for Primary Industries said tracing work from these farms was under way.

This week, four previously infected farms had their IP status lifted, MPI said.

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The farms were depopulated, cleaned and disinfected. Following this, they completed their 60-day stand-down period.

MPI's Recovery team was assisting the farms with returning to normal operation.

There are 37 active IPs - seven in the North Island and 30 in the South Island. Canterbury has 19.

MPI Director of Response for M. bovis Geoff Gwyn this week encouraged farmers affected by the cattle disease to speak up.

"That's the way you get the best feedback ... we won't shy away from the commentary, in fact I encourage it because it's people's critique of you that actually gets you to improve. We're dealing with people's lives so we can always be better."

"My greatest fear and the thing that keeps me awake at nights is those silent sufferers out there. Those people that might be under our regulatory control and for all our best intentions we really don't know how well they're faring ... believe me I worry a fair bit about the impact we have on people's lives."

Gwyn said spring milk test results were looming which would be a "significant moment," for MPI.

"Ultimately once it's complete around November we'll know whether there's any undetected clusters of farms out there."

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Gwyn said now was the time of year when dairy farmers started to bring on service bulls and some were worried that these animals may be from a farm infected with Mycoplasma bovis.

"We're comfortable we've got the properties of risk locked down," Gwyn said, "So I would say it's normal farmer risk management practices.

"Understand the origin of the animal, understand the health status of the herd it came from and it's good practice to separate the animal for seven days to access its health status before you mingle it with any other animals."