When Coca-Cola Amatil boss Terry Davis fronts the group's AGM Tuesday morning he will be asked to explain why a bottle of Coca-Cola costs up to two-thirds more in Australia than Asia.

It is a question one of CCA's main customers, Coles, asked at the weekend, as the supermarket chains get to the pointy end of negotiations with the Australian Food and Grocery Council (AFGC) to develop a voluntary code of conduct to curb the power of the supermarkets.

It echoes questions from Woolworths last week about global suppliers, who, they argue, earn fat profits by charging Australian retailers higher wholesale prices than they charge retailers in other countries.

The questions demand answers, particularly given CCA lost millions of dollars in revenue in 2011 when some of its key brands were taken "off promotion" at Woolworths for five months in a dispute over new trading terms with the chain. At the time some analysts estimated that the dispute translated into a loss in sales of 5 million cases.

Earlier in 2011 Foster's was forced to pull tens of thousands of cartons of VB, Carlton Draught and Pure Blonde from Coles and Woolworths after learning of a blitz promotion to sell beer at $28 a carton, at what they argued was well below cost.

At the time, a spokesman for Foster's said the company withheld supply to protect equity in its brands. ''We take loss leading of our brands very seriously,'' he said. ''We will consider our options when it comes to individual cases of loss leading.''

Since the $1 milk wars, followed by bread wars, beer wars and other wars, the supermarket chains have come under heavy scrutiny, resulting in the competition watchdog launching an investigation into allegations of misuse of market power.

The chains vigorously deny the allegations but in February this year when Davis called for a national debate on "aggressive supermarket practices", including selling popular products below cost as ''loss leaders'', it put the debate back on the political agenda. The reason? Suppliers rarely openly criticise the chains for fear of retribution.

It is against this backdrop - and as code discussions reach a finale - that Coles and Woolworths have cleverly shifted the debate from themselves to the big multinationals and the prices they charge in Australia compared with other countries.

It is seen in some quarters as a cynical attempt to pressure the AFGC to agree to a voluntary code that doesn't deal with the growing vertical integration of the chains. The supermarkets want a code but they don't want it to be too onerous when it comes to mechanisms to deal with vertical integration.

Options being canvassed include structural separation of the supermarkets' private-label-development marketing teams from their branded buying arm so that the buyers don't share information. Under such a model, retailers would be required to bid for shelf space for private labels in the same way as branded suppliers.

The clock is ticking. If a voluntary code can't be agreed, then a mandatory code could be forced before the federal election - or an even worse fate if the Coalition is elected.

Woolworths met the AFGC on Monday and, according to a spokeswoman from Woolworths, it was "very productive". She said Woolworths agreed on the broad principles and would continue to work towards a voluntary code.

It will now compile a written submission that will form the basis of the next round of discussions.

For its part Coles is putting the finishing touches to a more thorough price comparison on various countries and products with a view to putting its findings to the AFGC as part of its negotiations on the voluntary code.

It is an issue that Coles boss Ian McLeod first raised in the media in 2009 but stopped short of naming the main offenders.

Coles conducted a price comparison late last year of a few products sold in Thailand and Australia and found big discrepancies. For instance a two-litre bottle of Coca-Cola cost 54 per cent more in Australia than Indonesia in November 2012. It found CCA's Powerade sports drink was 87 per cent more expensive in Australia and its Vitamin Water was 83 per cent more expensive.

The price comparison recorded showed a similar pattern. This included some products from Proctor &Gamble, Nestle and Unilever products.

Proctor & Gamble's Oral B All Rounder Toothbrush one-pack was 38 per cent more expensive in Australia than Thailand and its Pantene shampoo was almost 50 per cent more expensive.

But when it comes to numbers, nothing is ever straightforward.

A report by Macquarie Equities in November, 2012, suggests the biggest contributor to profit growth for Coles and Woolworths over the past five years is the transfer of profits from suppliers to the retailers.

"A popular positioning is that Australian suppliers over-earn relative to overseas peers or parent companies. There seems little evidence of this with local suppliers' gross profit margins declining around 600 basis points over the last five years."

Over to Terry Davis and others for an explanation.

Grocery Chains Put Heat On Suppliers