An outbreak of equine herpes has hit a Waikato thoroughbred stud farm, leaving seven horses dead - and an expert says it could spring up in other farms.

Thirteen horses have been infected with the Equine Herpes Virus Type 1 (EHV-1), seven of which have been euthanised, a Ministry of Primary Industries (MPI) spokeswoman told NZ Newswire.

The outbreak is contained within a small part of one Waikato farm and the owner's 170 horses have been put into a voluntary quarantine for 21-28 days, MPI principal advisor Andre van Halderen said.

He said the EHV-1 is an endemic virus in horses, but it's the first time the rare neurological effects from the disease have been found in New Zealand.

The neurological effects have been known to occur in North America, Europe and Australia.

Van Halderen said there had been five clear days with no more cases, but there was no way of knowing if the farm was in the clear - and it was hard to pin-point what triggered the particularly severe outbreak.

The virus, which cannot transmit to people or other animals, is often dormant in horses, and becomes active when triggered by events including abortion and respiratory problems.

Van Halderen said it was possible the neurological effects could be triggered in horses in other farms although he said it was a rare syndrome.

He added that horses were often euthanised in the early stages of the virus for welfare reasons as the animals became more prone to other diseases.

MPI is investigating what caused the outbreak.

On its website Equestrian Sports New Zealand said equestrian activities would go ahead as planned.