I know there is a VAT module which probably does the same job as this, but GST seems a lot more simple then VAT, so I thought I'd just put together a simple module for it. All this module does is allow you to put in your percentage of GST, then on the review order and order it shows how much GST was included in the order. That way you just include the GST in the price of the product, and this will show the customer how much GST was included in the price. It also calculates the GST on the shipping.
I'd like to have a few more options, but I'm not sure what would be needed. One thing that it doesn't do, is remove the GST amount all together if someone from outside the country is ordering. I think that is the most major thing to add in. Any other suggestions would be good because I'm not an expert on GST and I don't know the differences between GST in Australia and other places.