Roundup manufacturer Monsanto was successfully sued by a groundskeeper in the United States for $289million after he claimed the company's glyphosate-based herbicides caused his lymphoma.
Green MP Eugenie Sage has said she will ask for Roundup to be put on the Environmental Protection Agency's list of hazardous substances up for reassessment.
The active ingredient, glyphosate, is already on the list.
University of Otago toxicologist Belinda Cridge said the International Agency for Research on Cancer (IARC) had found glyphosate was ''probably carcinogenic''.
''It is not a full risk assessment that would also take into account possible exposure rates and wider factors.
''But, yes, there is evidence that glyphosate causes cancer in humans,'' Dr Cridge said.
The EPA has approved about 60 substances containing glyphosate, which come with safety instructions.
Glyphosate-based herbicides are used across Otago.
The Otago Regional Council alone uses up to 4000 litres of Ken-up 360 every year on drain, willow and channel maintenance.
Dr Cridge said Roundup ''isn't great'', but said many alternative options were ''much, much worse for both people and the environment''.
During the Monsanto court case, one of the lawyers said that glyphosate had a ''synergistic'' effect with other chemicals contained in Roundup, but Dr Cridge said that conclusion was ''pushing the knowledge that we have to the limit''.
''Synergistic action is possible and a plausible mechanism, but currently it is unlikely to be fully tested and understood.''
Toko Spraying contractor Rod Murdoch said he had looked into using organic herbicides, but they were expensive - about $200 a hectare.
''If there were something else that worked, people would probably use it.''
Organic herbicide manufacturer Trish Goulter, of Goulter's Vinegar Products, agreed it was expensive to buy large quantities of natural herbicide, but like everything else, if people started buying more of it it would become cheaper.
Her product was already used by the Nelson City Council, she said.
