When you first register for GST, you are given the choice of how to account for claiming or charging GST.  This is called the accounting basis.

There are three options:

1. Payments (most commonly used method)
2. Invoice
3. Hybrid (very uncommon to use this method)

Payment Basis:

The GST Paid or Collected is based on the actual payments made and the actual payments received for goods or services, and is typically called a 'cash-based' system.

Invoice Basis:

The GST return is prepared based on when invoices are generated for customers/clients and when invoices are received from suppliers. The amounts are included in the GST return whether the invoices have been paid or not. 

Hybrid Basis:

The GST return is prepared based on invoices generated for customers/clients whether they have been paid or not, and the payment of invoices received.