Hi, I just went through this process myself and found it quite confusing.
I found that setting up an Australia wide tax rate of 10% for the “Standard Rate” tax class and also another Australia wide tax rate of 0% for the “Zero Rate” tax class solved my issues.
The issue I was having was that the cart total and calculated GST were all correct, but the cart / checkout item price was always adding another 10% on top, despite the checkbox ticked for “Catalog prices defined including tax”.
So, with both tax class rates defined, I set my products to the zero tax rate, which keeps the cart / checkout items at the correct price. The 10% GST Standard Rate is then applied at the cart level it seems.
In all honesty, I’m not entirely sure why / how this method works, it was an accidental solution I stumbled across whilst going through hours of trial and error.
Hopefully this may help you in your quest ??
