Cattle illness mycoplasma bovis has been found at another South Island farm, the Ministry for Primary Industries says.

The discovery of the bacterial infection at a Van Leeuwen Dairy Group farm brings the total of number of farms hit so far to seven.

But the ministry says the new discovery is far from unexpected and the farm was already under restrictions.

"The nature of this disease is that it spreads between animals that are in close, repeated and prolonged contact," response incident controller Stephen Bell said on Monday.

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"We've said all along that we fully expected the possibility for further farms within this enterprise testing positive."

He said the discovery showed the current intensive testing regime was working.

"Testing like this, over two to three months, gives us the confidence we need that we have definite results for each farm," he said.

"It has meant there has been a long period of disruption and uncertainty for farms that are being tested but we have to be absolutely thorough in diagnosing positive and negative farms."

Since Mycloplasma bovis was first confirmed in New Zealand on July 22 at the dairy group's farms, about 26,000 tests have been carried out at MPIs Wallaceville lab.

All the current infected properties have direct links to each other through animals moving from one farm to another and disease passing between animals in close, repeated and prolonged contact.

"District-wide surveillance in Waimate and Waitaki has been part of this. Bulk and discard milks were collected from approximately 260 farms in the area and tested," Mr Bell said.

There has also been a nationwide testing programme and the ministry hopes to have a final result by mid October, before considering a number of options, including herd eradication.