There's two ways you can go about this. The simplest and most reliable approach would not have the GST included in the item price but, rather, added as a separate Tax amount in the buyer's cart:
- Log into Seller Admin;
- Click Sales Tax/VAT Calculation;
- Tick "Tax buyers from country" and select Australia;
- For the Sales Tax/VAT Rate, enter your GST rate;
- Click Submit to apply changes;
- In the settings of GST-subject products, tick the Sales Tax/VAT setting.
However, if you need to have GST included in the item unit price, you could just mark up the item's Price setting by that amount. Then for non-AU buyers, you could set up a Discount Code for each taxed item, which deducts that tax markup amount from the Item Unit Price, and instruct non-AU buyers to enter that code in their cart. Unfortunately, with this approach there would be no way to apply that discount automatically for non-AU buyers, nor any way to prevent AU buyers from applying it. If you could set up separate sales pages for non-AU buyers, it's possible to add the Discount Code to the button code, so the discount would be automatically applied for anyone who clicks those buttons -- this help page explains how: