Justin here - I can help shed some light on how to setup Australian taxes.
Usually, Australian prices are listed with the tax included in the price, and there are some changes to be made to ensure you're listing GST on your website and notifications.
I have a guide here that can assist you with making the necessary changes. Bear in mind, we have updated our admin since then, but you can find most of the items under your Settings menu.
Every country will have it's own tax rate, and as long as you're based out of Australia, New Zealand customers should never be charged taxes.
If you had opened a location in New Zealand and sold the NZ customer an item from your NZ location, then that 15% would apply.
Nothing to worry about there, but there are some great points in the doc I linked above. :)
Hope that helps!
I have recently set up to charge GST on my store as well, however I also sell internationally. My prices by law need to show the cost plus gst as the sales price but when a customer buys from overseas, at the moment they are paying the taxes as well. Of all the threads and forums about this, there doesn't seem to be a work around on this important issue. Any recent developments on this?