Given their market dominance, it doesn't pay for suppliers to get offside with them, or refuse any requests, no matter how unreasonable they may seem.
In practical terms, most suppliers are faced with the prospect of capitulating to their demands, or going out of business. That's why there's an expression called "cliffing" in the grocery retail trade. It describes the situation suppliers sometimes find themselves in when negotiating with a supermarket. Either they agree to the supermarket's demands, or they jump off a cliff - that is, have their products kicked off the supermarket shelf.
The situation is made more difficult for suppliers because there is no transparency around how supermarkets operate; no rules as to what constitutes fair, or unfair, business practices, and no oversight or scrutiny of the way supermarkets do business with suppliers, to ensure they don't abuse their market power.
In Britain, an investigation into their grocery retail sector concluded supermarkets were engaging in trading practices that "transferred excessive risks and unexpected costs" to suppliers. As a result, the British Government has developed a Supermarket Code of Conduct which seeks to ensure supermarkets treat suppliers fairly and don't engage in unfair trading practices. The code is incorporated into every contract between large grocery retailers and their suppliers.
While relationships are still challenging, the very existence of the code reportedly knocked out some of the worst supermarket behaviour overnight.
The British Government has also agreed to set up a Groceries Code Adjudicator to monitor and enforce their code. Suppliers can approach the adjudicator if they believe they've been treated unfairly, and complaints will be handled anonymously.
I believe we need the same thing here.
I have spoken to numerous suppliers who claim supermarkets use a variety of tactics that can make life very difficult for them. They can penalise suppliers if they supply another supermarket chain, by giving them poor shelf space or even threatening to delist them.
They can put rival products on the bottom or top shelves and their own home brand products at eye level. They can require suppliers to pay for the cost of "specials" and promotions, and to pick up more and more costs such as freight, warehousing, food handling and marketing.
These sorts of tactics can be devastating for small suppliers, in particular, and put them under severe financial stress. But they have no recourse, even if they believe supermarkets are engaging in anti-competitive trading practices.
That's why, if the Government wants to support our struggling food sector and protect its members against unfair trading practices, it should follow the lead of Britain and develop a supermarket code of practice here.
The code would set out fair rules for suppliers, bring some transparency and clarity to the supply chain, and ensure that suppliers weren't unfairly treated.
It would benefit consumers, too, because if small suppliers are forced out of business by anti-competitive practices, this will have a detrimental effect on consumer choice as well as on competition in the sector.
* Sue Kedgley was a Green Party MP from 1999 until 2011 and campaigns for safe, healthy food.