If you want a reliable answer, then go to
www.promolaw.com and hire an attorney who specializes in that field. The idea that you are going to find someone who knows the law in Australia and any other jurisdiction in which you might offer prizes, and that someone with this amazingly precise expertise is going to offer you a question on the spot for $20 is, um, unrealistic.
Any other lawyer, and even one moderately familiar with online constests (I've dealt with two) is going to have to do at least some research, and still engage another attorney for questions offshore.
The proposition of "I want to give things away for free to adults who have consented to receive those things" appears, at least on the surface, to be fairly innocuous as a general proposition. To the extent that you are going to establish conditions on whether you give things to people, or to require something of them in order to receive things from you - that's where things can get sticky, and in which you run into a patchwork of local regulations based on experience with people running "contests" out of motives undoubtedly less pure than your own.
When you say something like "member of the month" or "most interesting post", there is always going to be someone who feels that their post is the most interesting, or that they are the most valued member that month, and if they don't get their freebie in exchange for the deal they thought they were making, then they will be sure to let you know.
Take dnforum.com - a fairly innocuous idea of "I want to have a place where people can offer things for sale, and interested buyers can communicate with them". Now, in a thread in the legal forum, we have someone who is screaming bloody murder about how the operators of dnforum are engaging in some heinous illegality by not policing what people choose to buy and sell.
It's a big world, and there will always be someone to find fault with what you do. As a lawyer, it would be very easy to take $20 bills in exchange for telling people "No, if you do that, you'll get into trouble. Don't do that" all day long in response to just about any question. But the real world is one in which decisions need to be made by balancing risks and rewards, and in which an assessment of your total picture is a necessary component of any decision about an answer that is appropriate to your perceptions of risk and reward. That doesn't happen for $20.