One of two Waimate district farms, which were placed under restrictions this month, has tested positive for the cattle disease Mycoplasma bovis.

Testing is ongoing on the second property, while a third farm has been placed under restrictions for the time being.

All three farms are in the same district, and a couple of them neighbour Van Leeuwen Dairy Group properties, where the disease was detected in August.

The Ministry for Primary Industries confirmed it's the eighth farm to test positive for Mycoplasma bovis, and the number of South Island cattle being culled in an attempt to eradicate it has now risen to 5,000.

MPI Incident Controller David Yard said the latest affected farmers in Waimate are devastated their animals will be killed.

''Those property owners have approximately 1000 further cattle on it. It needs to be done in conjunction with the meat works and the owners. There is a lot of operational planning that needs to happen.''

He assured farmers that if Mycoplasma bovis is present on their properties, it will be found.

"We still have a lot of work to do ascertaining the source of infection at the confirmed infected property, and building a picture of animal movements between all three farms and possible other farms," Mr Yard said.

The cattle disease is spread through close contact between animals, and through the direct movement of cattle between properties.

It causes udder infections, pneumonia and arthritis.