Trish Pearce, executive adviser to the Equine Health Association, said there were confirmed cases of the virus in 12 mares, six of which died.
Thirty other mares that had been in contact with the infected dozen were segregated from the rest of the population and were being monitored for any possible symptoms.
Mrs Pearce said the virus was most likely to have arrived in the country with an imported horse.
So far, only horses three years or older had been affected.
Symptoms varied in infected horses, but many found it hard to keep their balance, had trouble urinating and were eventually unable to stand, Mrs Pearce said.
Horsetalk.co.nz editor Robin Marshall said the flow of information had been slow.
"People want answers about where and how long it's been known," she said.
"It can be quite economically damaging and can be fatal to the horses."
Andrew Coleman from the ministry said the owners of the infected horses had done everything right, including quarantining the animals at risk.
Mr Coleman said ministry officials were confident the outbreak was contained on the one property. They were checking regularly on the horses during the continued quarantine period.
A New Zealand Veterinary Association spokeswoman said all New Zealand vets were given information about the virus on Friday.
Cambridge Stud owner Sir Patrick Hogan said the outbreak wasn't on his property but he was satisfied the problem was being contained.
"I am confident there are people in the industry who have the expertise to deal with it ..."
Mark Chittick, owner and general manager of Waikato Stud, questioned the processes that allowed an imported horse carrying the infection to get into the country.
"Obviously that needs to be looked at because we have relatively few equine diseases compared to other countries around the world and it's very important that we keep it that way."
He said transporting horses around the country was the biggest issue in the virus spreading.
"We won't be shifting any horses."
