New Zealand's Environmental Protection Authority says it still believes Roundup is safe to use, despite a US court ruling the weed killer contributed to a janitor's cancer and awarded him $NZ440 million.

Use of a popular weed killer in New Zealand has been called into question after the US court ruling.

A San Francisco jury yesterday ordered Monsanto to pay US$289 million ($439.1m) to a former school groundskeeper, on the basis its product Roundup had contributed to his cancer.

The lawsuit from Dewayne Johnson claimed Roundup caused his non-Hodgkin's lymphoma, which Monsanto denies.

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The company insists hundreds of studies have established that the active ingredient in Roundup - glyphosate - is safe and reportedly plans to appeal the decision.

Dr Fiona Thomson-Carter, general manager of New Zealand's EPA's Hazardous Substances Group, said the authority was aware of the US court ruling.

"The EPA has not been involved in the court case, and is not aware of any specific evidence which was used in court.

"There is no change to the science behind our current position, which is products containing glyphosate remain safe to use when you follow the instructions on the products label."

However Associate Environment Minister Eugenie Sage said New Zealand's EPA already had glyphosate on the list of hazardous substances for a chief executive-initiated reassessment under the hazardous substance reassessment programme. Glyphosate is the active ingredient in Roundup.

"I will be asking the EPA CEO to consider adding Roundup to the reassessment list in the light of the US decision," Sage said.

"Any decision to add it is the EPA's not the minister's."

Former South Otago organic farmer Graham Clarke welcomed the US judgment, calling it a "fantastic result" but said "it won't be the end of it of course".

Clarke has about a decade of experience in organic farming and only recently stepped down from his national role at the Soil and Health Association.

He predicted hundreds of thousands of complaints would emerge after the landmark case.

Arguably, Roundup's biggest strength - when using it in farming - was that it would kill anything you put it on, he said.

But it had been presented as something that would only affect plants, he said.

There was now a "growing awareness that that's a load of bollocks".

"We need to get on and find a better way," he said.

Federated Farmers national vice-president Andrew Hoggard said most of the scientific research, with the exception of one report by the WHO, said it was safe to use.

"I don't think the science is robust enough to say, you know off that one study by WHO, that it's bad.

"I was in Europe not so long ago and it quite a topical issue over there. They very nearly banned it."

Hoggard said he would be very concerned if the product was banned in New Zealand and added that there was not a suitable replacement for farmers available.

"My understanding is that you would have to go back to chemicals that are a hell of a lot more toxic."

It was an effective product for spraying out paddocks between pastures, and if not Roundup something with the same active agent - glyphosate - would need to be used, he said.

"It's one of those trade-offs in life," he said.

Farmers should use common sense and not apply the spray on windy days, he said.