Illegally imported drugs could be behind the breakout of mycoplasma bovis in New Zealand as authorities hunt for the way the disease entered the country.
Cattle on all 28 infected properties were to be slaughtered, with more than 22,000 culled in total.
The Ministry for Primary Industries (MPI) said the disease was not widespread but the affected farms were connected by animal movements.
While the authorities have not confirmed the source of the outbreak, New Zealand news outlets were reporting two searches on properties in the North Island carried out in late March were related to veterinary businesses importing illegal veterinary drugs.
Another search in Southland involved a farm.
There has also been speculation about illegally imported contaminated bull semen.
MPI manager of compliance investigations Gary Orr confirmed the searches related to potential breaches of legislation connected to the M.bovis response.
"We recognise there is strong interest in the rural sector concerning how M.bovis may have entered New Zealand," he said.
"We will ensure the outcome of these investigations is communicated to farmers as soon as we are able to provide that information.”
The ministry was unable to provide any further comment while investigations continued.
DairyNZ chief executive Tim Mackle said the investigation into the properties meant they were a step closer to understanding how the disease entered New Zealand, which would help prevent it happening in the future.
“If any farmers have put their fellow farmers, animals and livelihoods at risk, let alone the New Zealand economy at risk, then there needs to be consequences, and that is what other farmers are saying to me,” he said.
“We are extremely concerned to hear that legislation may have been breached in this way.
“The biosecurity rules and laws are in place for good reasons, and this M.bovis outbreak is a sobering reminder of that.”