A royal commission into supermarkets' relationship with farmers would not be limited to the dairy industry, Agriculture Minister David Littleproud says.
Senior Nationals including Mr Littleproud are open to the prospect of a major inquiry after criticising supermarket giants for refusing to put a 10 cent "drought levy" on all brands of milk.
"You never say never. If we are to go down that track I would not limit it to dairy," the minister told the ABC.
"I think agricultural products more broadly and their relationships to supermarkets, if it was to go down a royal commission level, should be tested rather than just a narrow focus on milk."
Woolworths was first to move, putting a 10c a litre levy on specially labelled drought relief milk.
Coles argues it has committed more than $12 million to drought-affected farmers while also putting a 30c levy on its own brand of three-litre milk.
Aldi says it has accepted price increases from suppliers which haven't been passed on to customers.
Mr Littleproud is disappointed the companies have not responded with bigger commitments to his offer to work on helping farmers.
"They've just used it as a publicity stunt and candidly, I'm not overly chuffed about it," he said.
He reserved his most savage criticism for Aldi.
"The big German Aldi has done three parts of bugger all," Mr Littleproud said.
Farmers and the government are continuing to pressure supermarkets over $1 milk which they say is hurting producers.
"It puts a ceiling on the price and on the potential of a commodity. I'm not in favour of it. I think it's a deplorable thing," the minister said.
