You do not have to have a return ticket for your visitor visa to Australia. However, you do need to demonstrate that you are a ‘genuine temporary entrant’. One of the ways the Department likes to see this, is by you having a return plane ticket.
It is possible that if you don’t have a return ticket, you may be questioned about your intentions at the border. You will not be allowed in the country if it is suspected that you are not going to abide by your visa conditions or that you are going to overstay your visa.
If you elect not to purchase a return ticket, you will need to be able to demonstrate that you have sufficient funds to purchase a plane ticket to return home. You may want to carry a copy of your bank statement for this purpose.
If you are unsure of your exact travel plans, you could consider buying a flexible ticket.