CRM - TAX INCLSUIVE

Hi all, I have just registered this software today and am needing to get it functioning asap.

I have a question, I am hoping someone can kindly assist me with.

How does one add a GST (Australian Good's and service tax) to my invoicing and how does one make it inclusive.

I have managed to add the GST tax, however a $100 item (including GST) is coming out as $100 + $10 = $110 when it should be $100

Any advice would be greatly appreciated.

Daniel