Dear Toqual,
Do you mean get all your money back and keep the products - i.e. get them for free? If so I assume you are thinking of the Supermarket Code of Conduct to regulate supermarkets with computerised checkouts. This is an optional code that is adopted voluntarily by supermarkets. If they are a signatory to it, they must advertise this at checkouts. The code states that if an item scans at a higher price than what is said on the shelf or advertisement, you are entitled to receive the first item free and all later items at the lower advertised price. (This does not however apply to tobacco, liquor, items without a barcode or items of value exceeding $50 (advertised price).
However, general consumer laws state that if an item is marked with a shelf price, a consumer is generally only required to pay that advertised price. Where two different prices are displayed on one item, you are entitled to purchase the goods for the lower price.
Go back to the store and alert them to the error. If they are a large retailer I'm sure they will be more than happy to correct any mistake and refund any excess paid.