A new strain of parasite that can make dairy and beef cattle anaemic and to which they have no natural immunity is spreading rapidly throughout the North Island.
More than 100 farms have confirmed cases of the new type of theileria, which is spread by ticks and was first detected in the country last year. It is widespread in Australia.
The disease can make cattle lethargic and in extreme cases even kill them.
About 700 beef and dairy cattle have died on more than 100 farms and many more are sick.
The Ministry for Primary Industries says those numbers may be the tip of the iceberg and is urging farmers to be vigilant.
Its modelling sugests that if there is a confirmed case on a farm, then farms within a 5 km radius are also affected.
MPI principal advisor of animal disease response Andre van Halderen says the ministry has been investigating anaemia in cattle since August last year when a lot more cases began emerging.
He says provisional results from samples collected around the country suggest the disease is far more widespread than initially thought.
It has been confirmed in 20 local authority areas in the North Island and two in the South Island from only limited testing.
Mr van Halderen says the ministry is moving to get more medicine to treat the disease into the country and has issued a special import certificate for the strictly controlled drug.
Waikato farmer John Bluett says he discovered one of his cows had theileria last week.
He says he recognised it because he'd been on a farm that had had an outbreak of theileria the day before, so knew the symptoms.
Mr Bluett says a blood sample taken by the vet confirmed it and the cow has since had a blood transfusion and is now being milked once a day and is coming right.
He says if farmers suspect their cows may have theileria, or the symptoms are like milk fever, they should check their genitals and if they're white, call the vet.
The new strain of theileria has been detected as far south as Canterbury.