The confirmation of Mycoplasma bovis on two Ashburton properties this week, with a third under investigation, wasn't a surprise, says the Ministry for Primary Industries.
MPI picked up the properties through its background Bulk Tank Milk (BTM) surveillance programme, that is run though all dairy herds around New Zealand.
It had been expecting to detect the cattle disease during spring more than at other times of the year, due to the stress of calving, director for the Mycoplasma bovis programme, Stuart Anderson said.
"This is the time of year with heightened stress, if there's M.bovis out there in animals, we are more likely to find it" Anderson told The Country's Rowena Duncum.
It was understandable that the return of M.bovis would cause "some nervousness" in Ashburton, but people shouldn't worry, he said.
"We aren't looking at a widespread outbreak here."
The three properties currently affected by M. bovis were detected in August, and there had been no new cases found in September in Ashburton so far, Anderson said.
Nationwide testing had "made progress, but the job was not yet finished" Anderson said.
"We are no finding lower numbers of detections or traces that we need to look into and we're working a lot hard to find less, so we are confident in the progress of the programme and where we're at."
"There is still some work to be done but the numbers are heading in the right direction for the programme eradication effort."
Monthly background BTM testing, along with a stronger tracing system, meant that MPI had much higher confidence in finding any cases of M.bovis that may be still out there, Anderson said.
For example, the farms currently affected had initially tested negative, but monthly background BTM testing picked up a positive M.bovis result after the animals had come under stress, Anderson said.
Testing positive for M.bovis was also stressful time for farmers, and MPI was working closely with those involved to help them cope, Anderson said.