I am a European citizen and I have already got a visa to Australia for 3 months. Should I buy a return ticket in advance or can I buy it on my way back so I will just buy a single ticket for now?
Two responses, for depending on why you ask.
If you ask because you want to know if it's legal, then yes, you can enter on a single way ticket. See the advice on this web page from the Australian High Commission in the UK that specifically references single vs. one way tickets.
However, if you ask because you want to know if it's cost effective, then no, buy your return ticket. Two singles will almost always be more expensive.
The general law anywhere in the world is that you should be able to prove that you're leaving the country you're about to visit as a tourist. It's enforced differently across different countries, airlines, borders etc.
The safest option that still doesn't require you to have a return ticket is to book a valid flight with an agent that doesn't cost anything to cancel. Some agents would do it for free. There are even services that do it online, issuing you a valid ticket and canceling it within 24 hours.
Another option is showing some return flight you're not on. Make sure the flight exists and its times are correct. Another option is not having anything and hoping you won't be forced to buy an onward flight at the last minute as a condition to letting you on the place.
Good luck :)
What I always do is get a fake printed return ticket. The easiest way is to buy the return ticket at expedia.com, download the PDF with the e-ticket and immediately ask for cancelling the ticket (You can do it via the website up to 24h after the purchase with no costs and no hassle at all). Remember, the immigration officer has no access to the airline system in order to check the authenticy of the ticket.