A new strain of parasite which makes cattle anaemic has invaded over 300 farms - many of them lucrative dairy farms in the Waikato - and biosecurity checks are finding about 20 new infected herds each week.
The Primary Industries Ministry says 327 farms are so far confirmed as infected, and about 20 a week are returning positive tests for the parasite theileria ikeda, which kills about 1% of the cattle it infects and makes a third or the rest ill.
It's still waiting for the laboratory test results on samples from a further 20 suspected incursions.
The majority of farms that have been hit by theileria ikeda are dairy farms in Waikato and the ministry says nearly all the outbreaks are occurring in areas known for the cattle ticks which spread the parasite.
Ikeda is a new strain of theileria which was first detected in Northland in 2012.
The ministry says it's posing a severe challenge to the cattle industry and that New Zealand urgently needs to develop tools to monitor herds for the pest.