A judge will decide whether supermarket giant Coles deliberately misled customers with its "Down Down" price campaign in a landmark ruling ending a high-stakes legal battle.
Due to be delivered on Thursday morning by Justice Michael O'Bryan, the decision could change the way supermarkets operate and see the $28 billion company hit with nine-figure penalties.
The Australian Competition and Consumer Commission brought separate but similar cases against the country's two dominant supermarket chains with a Woolworths suit awaiting judgment at a later date.
Both cases allege the supermarkets misled consumers by increasing prices for a short time before lowering them to above the original price and marketing it as a discount.
In one instance, a can of Nature's Gift dog food was on sale for $4 nearly a year before the price was briefly increased to $6.
It was then sold under a "Down Down" ticket for $4.50.
The watchdog identified hundreds of products on the Coles campaign and a similar "Prices Dropped" push from Woolworths that followed a similar formula.
During separate hearings, lawyers for the supermarkets argued prices increased due to inflationary pressures and the discounts were genuine.
Melbourne University consumer law expert Jeannie Paterson said the outcome in the case against Coles was too close to call.
"People feel aggrieved there was this short-term price increase before the price was dropped - but, strictly, the marketing was accurate," she told AAP.
If Coles was found to have misled its customers it could mean "colossal" fines for the retailer and change the way all supermarkets did business, Professor Paterson said.
"Even if Coles wins, we're going to see a great deal of cynicism about promotional offers and pricing," she said.
ACCC barrister Garry Rich SC argued during hearings that Coles was trying to avoid losing customers by disguising price increases as discounts.
"Why on earth are you telling your customers your prices are going down? They're not," he told the court.
But Coles' barrister John Sheahan argued "ordinary, reasonable consumers" knew that prices generally trended upward due to inflation.
The consumer watchdog's case could be hurt by the fact that very little evidence was brought about shoppers' habits and their understanding of discount campaigns, Prof Patterson said.
A ruling against Coles would be instructive - but not definitive - when predicting the court's decision in the Woolworths case, she added.
"There are some differences in the evidence that was brought in the two cases," Prof Patterson said.
"Woolworths will be looking very closely."
