Discount not applied to total after tax

Hi

My role discount is being applied to the order subtotal correctly, but then my GST is being calculated against the original order subtotal, not the discounted subtotal.

So for a 20% discount and 10% GST I get:

Subtotal: $100
Discount: $20
Subtotal $80
GST: $10 (Should be $8)
Total: $90 (Should be $88)

Anyone with a fix?

Thanks

Glenn