I had the same problem where the tax was being added twice.

I realised it was because I also had an "All Zones" within the Australia zone as well as each individual state.

I originally had only "All Zones" for Australia since GST is charged everywhere. But some interstate customers weren't getting charged GST, so I created a zone for each state.

Now I have noticed that the GST is not being calculated correctly: A purchase of $130 says that GST is $13, but it should be $11.81 and I haven't been able to find any posts where this is resolved.

Would changing the tax rate to 9.09% be the most straight forward solution?

$130 / 11 = 11.81
$130 * .0909 = 11.81