A group of unions has descended on Parliament House, calling for a royal commission into the fruit and vegetable industry, claiming it has the worst exploitation of any workforce in Australia.

The federal government says it will not support a royal commission, and despite acknowledging "issues in the sector", the National Farmers' Federation says the call is "another example of union grandstanding".

The Australian Workers' Union, Transport Workers' Union and Shop Distributive and Allied Employees' Association have formed an alliance claiming to represent each sector of the retail supply chain.

It says several state inquiries, academic studies and the damning Harvest Trail Inquiry by the Fair Work Ombudsman have not done enough to clean up the horticulture sector.

"From violence to harassment, to passports being taken off, to forced slavery, to 76 workers living in a single house, forced to work 12-hour days, seven days a week, " AWU National Secretary Dan Walton said.

"Enough is enough. We need a royal commission into this industry. We need to shine a spotlight into the deepest and darkest corners of it, to clear it up once and for all," he said.

Farmers, government and opposition won't support call

In a statement to the ABC, the National Farmers' Federation said it acknowledged there were issues within the sector, but it would not support a royal commission.

The NFF referred to several recent inquiries into farm labour, including the Migrant Worker Task Force, the Fair Work Ombudsman's Harvest Trail Inquiry, the University of Adelaide's two-year investigation led by Dr Joanna Howe.

"A royal commission would not uncover any other material issues that these reports have not already done so," it said.

"Industry and government energies would be better spent on establishing national labour-hire regulation, promoting accreditation programs, promoting the new Commonwealth initiatives to assist growers and employees to understand their rights and obligations, and engaging with the soon-to-be-completed National Agriculture Workforce Strategy."

That strategy, prepared by the National Agricultural Labour Advisory Committee, was handed to the government last October.

Agriculture Minister David Littleproud said the government promoted fair work practices across the horticulture industry.

"The Australian government has zero tolerance for worker exploitation and has taken steps to strengthen the Fair Work Ombudsman and boost penalties," Mr Littleproud said.

"Instances of poor practices hurt the reputation of the industry and generalisations of the industry is not accurate or productive," he said.

Shadow Agriculture Minister Julie Collins would not say if the opposition supported the unions call for a royal commission but said she was concerned about the "poor working conditions of some fruit pickers".

AWU calls for floor in piece rate payments

The Australian Workers' Union is also calling for a change to way workers are paid piece rates when working on farms.

Secretary Dan Walton said piece rates — where workers are paid according to the amount of produce they harvest — were one of the few parts of the Australian workforce "where you're not guaranteed a minimum rate of pay".

Mr Walton claimed the AWU proposal would ensure a minimum rate of pay for those on piece rates.

"We're making an application to the Fair Work Commission, to say that if you're on piece rates and you do 10 hours a day, you should at least get the equivalent award rate for those 10 hours,"

The matter is listed for hearing at the Fair Work Commission in July.