The full bench of the Fair Work Commission on Tuesday began hearing from witnesses about an application to force farms to pay at least the minimum casual pay of $25.41 an hour.

Currently, workers can be remunerated based on the amount of fruit picked or vegetables harvested.

But the Australian Workers' Union says the complexity means some farm owners can take advantage of workers and pay them a few dollars per hour.

"Piece rates (make) it easy for vulnerable workers to be cheated, ripped off, and exploited," national secretary Daniel Walton said.

"An hourly wage floor would make it much easier for a worker - even a disadvantaged, vulnerable worker - to know if she's being ripped off."

The union presented the commission with 2016 research published in the Journal of Industrial Relations that suggested the average piecework picker was paid $11.69 an hour.

Piecework pickers were also more likely than colleagues on hourly rates to carry excessive loads or work in extreme heat.

Philip Gourlay, the AWU's organiser in Mildura on the Victoria-NSW border, estimated he received 20 to 30 complaints a year about pay rates and "regularly through harvest periods".

While he could speak to farm owners or otherwise help workers, the majority moved from area to area, he told the commission.

British citizen Sophie Blake told AAP she was paid about 15 cents per tree for pruning and cleaning up afterwards.

"It just feels a bit like slave labour because you definitely know you're worth more than that," she said.

The National Farmers' Federation said piecework rates incentivised and awarded productivity.

Chief executive Tony Mahar was concerned introducing a minimum floor price would further shrink the pool of suitable workers.

The hearing runs until Friday.

Australian Associated Press