1. Add an ABN to the provider practice you are billing for

From the Accounting screen in Medilink, go to File >> Change Details >> Provider Details (or press CTRL+O).

Click the '+' next to the providers name and select the practice to setup, then click Edit Practice

Fill in the ABN and ensure the Taxation Invoice checkbox is ticked.

2. Create invoice

Now when you create an invoice for this provider practice, you'll see an Add + GST button enabled when you're added items. 

There is also a GST Included checkbox which allows you to specify whether your item fee is inclusive of GST or not.