Vets have not yet disclosed what caused 120 cows to die suddenly on a south Taranaki farm on Tuesday, but nitrate poisoning has been ruled out.

The cows died after their water troughs had been topped up using a portable tank.

Within 30 minutes, cows began falling to the ground, a spokesman for the farm's owners said.

Vets were called immediately, but there was little they could do as some of the cows died quickly.

Associate Professor Dr Richard Laven, who lectures in production animal health at Massey University, told NZ Newswire the most likely cause of sudden death in such large numbers of animals is nitrate poisoning.

However, Dr Laven said Taranaki vet Stephen Hopkinson had ruled that out in this case.

"Nitrate poisoning is relatively easy to diagnose in a dead animal," he said.

"You can rule it out and be very sure that it's not."

Dr Laven said there are not many things that could kill animals in such a short space of time.

Other causes of sudden death could include infectious diseases, toxins or lightning strike.

"The most likely thing is it's an accident, rather than something infectious," Dr Laven said.

The farm's owners are distraught over the deaths.

"For everyone involved it was heartbreaking," their spokesman said.

The deaths mean the farmers have lost almost a quarter of their dairy herd.

The Ministry for Primary Industries is working with the farmers, vets and Federated Farmers to investigate the cause of the deaths.