If travelling to the UK as a tourist on an Australian passport must you have a return ticket to gain entry?
No it is not compulsory BUT if the border control agent (immigration officer) suspects that you intend to overstay the maximum 6 month tourist stay he/she may refuse you entry.
Ways of proving you do indeed intend to leave (and not stay illegally) include a lett fro your employer in Australia that you still have a job there and are expected back within a certain time frame. Or you could simply have a return ticket. If you plan on buying onward tickets from the UK and continuing your big trip, explain that to the border agent, and it's a good idea if you can show proof that you have enough money in the bank to buy an onward journey ticket.
You will also be asked to prove that you are able to support yourself for the period you intend to stay in the UK (in other words, you're not going to work illegally).
The full list of rules and documents needed are here:
http://www.ukba.homeoffice.gov.uk/ about 6 years ago