Mycoplasma bovis has been identified in three new properties, bringing the number of infected farms up to six.
The Ministry for Primary Industries confirmed the findings today, saying the new properties had existing links to other infected farms, and that it was "an entirely expected pattern at this stage of response".
"We fully expect to find more infected properties as we continue our tracing and testing programme," MPI response co-ordinator David Yard said. "These detections are evidence of the programme working, not of unexpected disease spread.
"All detections to date have links to the original infected properties via animal movements and have been caused by close animal contact. What is encouraging is that, despite intensive testing, no adjacent properties have as yet been identified as infected.
"We have no evidence of any means of disease spread other than close animal contact, at this stage. This includes the disease having jumped fences - which our scientists and vets tell us is highly unlikely to occur," he said.