The main retail supermarket chains (Woolworth, Coles, etc) have a voluntary Code of Practice whereby if the price an item scans at is different from the price marked on the shelf, you get the item for free. There is no legislated position regarding pricing errors, and only the supermarkets use this code.
It basically comes under the law of contract: the retailer offers goods for sale, the customer agrees to buy, and the price is determined between them. At any time up to the completion of the contract (the exchange of the goods for money) either party is free to opt out of the contract. You don't have to buy it at his price, he doesn't have to sell it at your price. Price tags are an indication, not a legally binding offer - otherwise how could you ever negotiate a good discount?.