Farmers are urging a Senate committee to take a broader look at supermarket pricing ahead of its inquiry into the Coles-Woolworths milk war.

The Senate inquiry will tomorrow begin its hearings into Coles' and Woolworths' decision to slash the price of their own brand milk to just $1 per litre.

Last week on AM, dairy farmers and cornerstore owners said the policy could drive them out of business.

But the West Australian Farmers Association now says Coles in particular is using marketing tactics to drive down the price of pork, beef, eggs and bread, as well as dairy products, to unsustainable levels.

President Mike Norton argues Coles is using marketing tactics which are affecting other commodities.

"The marketing and pricing principles, Coles have made it well and truly known that they intend to apply them across and wherever they can on every product line that they buy into and sell through their stores," he said.

"So whilst dairy is the focus, it will apply wherever and whenever on every product line."

He says ultimately this will result in shortages and consumers will eventually pay more.

"You eventually break the industry ... that is why you need to support the branded products," he said.

Coles will not be appearing at the committee's hearings this week, and neither Coles nor Woolworths have put in a submission to the inquiry.

Coles spokesman Jim Cooper says the company will appear at a later date to explain its dairy pricing policy.

He says the company is standing by its marketing and pricing.

"We are very comfortable that our policy is giving great value milk to our customers and we've set our prices in such a way that we don't believe the pricing policy will have an adverse impact on the broader dairy industry," he said.

"I think there is probably a litany of people trying to create pressure on the back of some public interest at the moment without a lot of justification."

Independent Senator Nick Xenophon is on the Senate Economics Committee, which will begin hearings into those prices this week.

He agrees there is a broad problem but says the inquiry will be focusing on dairy for now.

"The risk is that the Australian dairy industry is at the tipping point of having mass walkouts from farmers around the country, particularly North Queensland, which is particularly vulnerable," he said.

"The issue here is to find out what needs to be done, what legislative solutions need to be in place and really Coles needs to explain themselves to the Australian people in terms of their conduct and behaviour."