Any price you are quoted should be a total price. If a vendor quotes you $X plus GST - simply say "no thanks" - what they are doing is illegal (I think most small businesses do this unknowingly, they simply don't know the law).
If a business is displaying prices to consumers, the ACCC (Australian Competition & Consumer Commission) requires the quoted price to be clear, accurate and not misleading to consumers. It should state the total price of the good or service as a single figure that includes tax and other additional charges, including GST.
There are a number of smaller vendors that may not even charge you GST, if their sales are under $75,000 they don't need to register, pay, or claim GST - either way it's not your problem.
The price they quote MUST be the final price inclusive of any taxes and other charges.
** don't expect the same to apply if you are a business - price quotes from one business to another can use '$x plus GST'