The Animal Health Board is trying to ascertain how a dairy cow came to be infected with bovine Tuberculosis in Northland - a region that's supposed to be free of the disease.
It's the first case of the disease to be found in the region in eight years.
The infected cow, which has now been slaughtered, belonged to a large dairy herd in the Kaitaia district.
The rest of the herd is now being tested to see if any more cows are infected.
But the Animal Health Board says the cow was born on the farm and there is no evidence of bovine tuberculosis infection in surrounding wildlife, such as possums.
Regional coordinator Frank Pavitt says it's most likely the disease arrived with an infected animal brought into the herd from outside the region.