The lead applicant in the Federal Court case, 40-year-old Queensland man Kelvin McNickle, allegedly used Roundup as a child while spraying weeds with his father and then during his adulthood through his work.

Mr McNickle was first diagnosed with non-Hodgkin lymphoma in May 2018 and underwent chemotherapy and radiotherapy for about seven months.

Not long after, doctors told him he was in remission.

Six weeks ago he was again diagnosed with the disease.

"We cover a gamut of ordinary Australians who have developed non-Hodgkin lymphoma through their exposure to Roundup," Mr Watson told reporters on Monday.

Federal Court proceedings that began on Monday are slated to run for nine weeks and determine whether Roundup contains a carcinogen before other issues in the case are dealt with.

The civil action has been launched against Monsanto, which produced the weed killer. Bayer acquired Monsanto in 2018.

Mr McNickle will not attend the hearing while he receives cancer treatment.

"The upshot is, we are confident that Monsanto did everything it could to avoid confronting the reality that this chemical was a carcinogen and did everything it could to obscure the science and to hide the science," Mr Watson said.

The cancer patients were ultimately fighting for "very significant" damages, he said. 

Roundup is still used in Australia and the Australian Pesticides and Veterinary Medicines Authority supports the company's claim glyphosate-based herbicides are not carcinogenic.

Bayer says glyphosate-based herbicides have been rigorously tested in hundreds of studies, and the weight of that extensive body of science confirms that glyphosate is safe when used as directed.

"Monsanto’s defence of the claim demonstrates Bayer’s ongoing commitment to supporting Australian farmers by ensuring innovative products such as Roundup continue to be available, advancing sustainable agriculture and protecting food security," it said in a statement.

The Federal Court case is expected to come down to the science behind glyphosate.

Barrister Andrew Clements KC, representing Mr McNickle, on Monday laid out three streams of evidence they will rely on to prove glyphosate and glyphosate-based formulations are carcinogenic: epidemiological evidence, mechanistic evidence and animal studies evidence. 

“The greater the duration and intensity of exposure to Roundup Herbicide and Roundup Biactive, the greater the increase in the exposed individual’s risk of developing non-Hodgkin lymphoma,” the class action lawyers said.

Mr Clements pointed to several studies over the decades that he said supported the claim glyphosate was carcinogenic.

He criticised Monsanto for consistently working to discredit every study that raised concerns about the ingredient.

"It's the same old approach: ignore the smoke, there's no fire here," Mr Clements told the court.

Barrister Steven Finch SC, representing Monsanto, said Maurice Blackburn's own experts disproved their claims.

They were also wrong to suggest they could rely on one of their three streams of evidence to prove their case if two of the streams failed, he said.

“We make no apology at all for making the majority of our attack on the reliability of the studies presented to Your Honour," Mr Finch told Judge Michael Lee.

The barrister suggested experts cherry picked information based on outcomes rather than merit to support the class action's claims.

The majority of people who were diagnosed with non-Hodgkin lymphoma developed it because of a spontaneous mutation, and there was no laboratory evidence to show glyphosate-based formulations caused the disease, Mr Finch said.