There is a lot of confusion regarding whether you need to be registered for GST and what this means. If you run a business, you will need an ABN, however, you don't need to be registered for GST.
According to the ATO website, you must register for GST if:
If you are under this threshold, then you have the option to register for GST. If you do voluntarily register for GST, then you will need to charge your customers GST (if it is a GSTable service/product) and you can claim GST on your expenses. Many businesses that are in the health industry where they don't need to charge their customers GST, but can claim GST back on their expenses, voluntarily register themselves for GST.
If you do not register yourself for GST, then your invoices should not mention the words "Tax Invoice". Instead it should just say "Invoice". Also, there should be no reference to the words GST. If your customer requires a tax invoice and you are not registered, you should state that there is no GST. This can be achieved by making a statement such as "Price does not include GST".
If you have not been registered for GST and have accidentally charged your customers GST, then you should refund your customers the GST component that they have paid you. You should also tell them that you are not registered for GST. If you are not registered for GST, you should check each to month to ensure you have kept under the threshold. If your turnover exceeds the threshold, you have 21 days to register for GST.