Hello @AndrewK,
If you haven’t already, I would suggest checking out @elisegaines’s awesome answer to a similar topic in the Community post below:
In essence, you’ll want to review your Started Checkout and see if it’s passing any additional prices such as one that does include the GST/tax amount.
If your Started Checkout event was not passing an additional price inclusive of the tax, another workaround would be using variables filters for math to calculate and display the product cost with the tax amount. This solution works great if you have a standard tax tax rate or if you’re passing an additional variable that is just the tax amount. I touched upon this a bit in the Community post below:
I hope this helps!
David