Today, we learned the new boss of the Australian Competition and Consumer Commission is investigating how the two retail giants use petrol dscount dockets.

And that comes on top of an even bigger inquiry - how Coles and Woolworth treat their suppliers.

The gloves are off and all eyes are on Rod Sims, current chief of consumer watchdog the ACCC.

• Big banks, bad service
• 'Pinch me' for a free offer
• Is this the world's toughest realtor?

Sims is starting a hell of a fight with a two headed Goliath - Coles and Woolworths.

Former head of the Australian Competition and Consumer Commission, Professor Allan Fels says the investigation is long overdue, but Sims can't wage a one man war.

"This is the moment for suppliers to step up or cop it forever”, he says.

Coles and Woolworths are more competitive than in the past 15 years, and there are claims their suppliers have been caught in the cross hairs.

"The effective duopoly that Coles and Woollies have is unprecedented in the developed world”, says IBIS World's Nahran Sivasailam.

"Essentially the corner shops and the butchers and the fish mongers and your local cheese makers; they've been muscled out to some extent simply because of the buying power than Coles and Woollies have.

So all consuming are these giants, even some suppliers feel it's smarter to suffer in silence than speak out.

"I think it's very sad that suppliers don't feel they can come forward and speak openly, that there will be bullying and intimidation. I think their fears are somewhat well grounded”, Professor Fels says.

He believes Coles and Woolworths loathe public humiliation, but also fear this new investigation could prompt Parliament to toughen up its laws and change the supermarket landscape forever.

"They are really squeezing the suppliers very hard by any standards of the past so it coul be now crossing the borderline between legal and illegal behaviour."

Since Wesfarmers married Coles seven years ago, paying $22billion for the pleasure, the family tree is all but groaning under the weight.

Apart from its supermarkets it counts Liqourland, Vintage Cellars, Officeworks, Bunnings, Kmart, Target, mining, insurance, gas and electricity companies as all part of the clan.

Woolworths has its supermarkets including Flemings; and its liquor stores Dan Murphy's, along with BWS, Big W, and Tandy and Masters.

Coles and Woolies are undoubtedly heavy weights in our supermarket landscape; Together they own more than 1600 supermarkets Australiawide.

Nahran Sivasailam says it’s a wind loss situation for consumers.

"Consumers are benefitting if you're looking at price. If you're looking at choice, private labels now account for about a one quarter of all sales, growing from about 13% five years ago. So that's 85% growth

There are concerns they may be misusing market power by discriminating in favour of their own house brand products.

Marketing expert Barry Urquart says 1 in 4 products in the average trolley is home brand.

"The objective of both Coles and Woolworths is to increase house brands to as much as 47 per cent of their sales."

Bundle up the basics; bread, milk, cheese, Coke and Scotch finger biscuits and swap them for housebrand and you pay a quarter of the price.

It's not suprising then that Coca Cola Amatil is calling for debate over discount tactics. It’s up in arms and in support of the ACCC investigation.

Of course, there will be denials about any wrong doing. Some may say they're just business savvy.

But with just two big chains, there's little surprise that accusations are flying from farmers, meat producers, dairies and market gardeners about getting screwed.

But as Malcolm Maiden, business columnist for Fairfax Media suggests, now they have to prove it.

"The crucial shift now is that [the ACCC's] Rod Sims says he's got enough info to believe that there might be something going on. Suppliers, of course may be concerned that if [they] start airing complaints about these big supermarkets, they may lose a source of sales for their products, which is just absolutely crucial to their survival.”

There are small competitors, IGA and Aldi and Cosco, are muscling in, but undoubtedly Wesfarmers Coles and Woolworths are the twin packs of power.

Other heavyweights Walmart and Tesco to date have stayed away, perhaps some say because it's all too hard to break in.

But when squeezing suppliers there's always the obvious defence.

"Another complication of course is that it's quite possible that some of this behaviour has been delivering lower prices in the supermarkets for ordinary shoppers”, Malcolm Maiden warns.

As an expression of self-healing the big two have started their own investigations into allegations by suppliers working on a voluntary code of conduct - but it's Rod Sims who will be the true decide in this epic battle.