Are my invoices tax invoices?
When you are registered for GST and charge it, the invoice is headed Tax Invoice. Otherwise it is headed Invoice.
Yes, when the business is registered for GST and the invoice charges it: it is headed Tax Invoice. If you are not registered for GST, it is headed Invoice and no GST is charged.
Set your tax number and whether you are Registered for GST under Settings, Business details. In Australia that is your ABN and 10% GST; in New Zealand, your GST (IRD) number and 15% GST.