I recently bought a meal at a restaurant and when I looked at the receipt later I noticed that it shows a GST2 charge of 2.5%. As far as I know there has only been one GST rate in Australia since it was introduced: 10%. Some items, like basic food, are GST-free, so effectively they have a GST of 0%. I've never heard of GST being charged at any other rate in Australia. Is this legal?
You are correct in that the GST is 10%. It never has been (and likely never will be!) 2.5%. You're also correct that some items (generally considered as 'essentials') are GST free. For example, the ATO includes 'most basic food' on its list of GST-free items. However, these GST-free items are completely GST free, and restaurant meals certainly don't class as basic food.
Perhaps interestingly, the wholesale sales tax when it was first introduced in 1930 was set at 2.5%, but it wasn't at that rate for long (see A brief history of Australia's tax system).
If I had to guess, I'd say this is probably a dodgy set up in this restaurant's point-of-sale system (there's also no such thing as 'GST2'). Hopefully their returns to the tax office are not as dodgy!