GST by law is supposed to be included in the price and can not be added after. The receipt you receive should show the full price of the item and how much of your total purchase was GST.

There are some grocery stores in town that list the pre-GST price on the items on the shelf and then charge you that plus GST at the counter. That is not legal if you check the laws.

I have also heard that some restaurants are doing the same. If you eat at a restaurant and they add the GST at the end to the menu prices, refuse to pay it.

I also add GST afterward and have had an audit. The Auditor did not mention this and every sales ticket, advertised price and X-cel sheet he looked at has GST added after the price.
I agree with you Simon its probably the law , but whats the point.

The point is that if you adverise a product for $10 the customer pays $10 - not $11.25 - so the $10 includes the tax. Where it gets picky and they might turn an eye is if you advertise $10 + tax and your billing and books are as you describe. It's not technically right but not so wrong as worthy of change.