How far Coles and Woolies' alleged dodgy discounts go

Documents filed with the Federal Court show new details of Coles and Woolworths' alleged fake discounts, which could be far worse than first thought.

It's through those documents the true breadth of the alleged scam is becoming clear, Nine's finance editor Chris Kohler says.

"This system for allegedly duping customers has now been laid out in forensic detail," he says.

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"And it seems to cover every aisle of the country's biggest supermarkets."

Coles and Woolworths are alleged to have hiked the prices of products before dropping the prices a little and slapping a discount sticker on them and calling them sales, according to the Australian Competition and Consumer Commission (ACCC).

Court documents filed by the regulator today claim that system was used for 600 products across those supermarkets.

For example, insect repellent was originally prices at $9 before rising to $13.50 and then discounted by $1 - that means the discount was 39 per cent higher than the original.

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That's what the regulator says is "misleading" customers and against the law.

Lemonade, dog food, noodles and biscuits are being put forward as some of Woolies' biggest fake discounts, ranging for 29-34 per cent higher than the original prices.

For Coles, it's claimed hand soap, coffee, soft drinks and pet food top their list, ranging from 33-36 per cent above original prices.

And despite these two supermarkets saying they're fierce competitors, the system they allegedly used across the 600 products on the list was almost identical.

Along with the court case, the ACCC is also conducting a report into supermarket behaviour, which is due out in February.