Well, Samhain was supposed to provide closure for family and friends of those deceased during the year and represent a symbolic death of the year itself. Of course it involved a certain amount of unease, as the veil between life and death growing thin is still a slightly worrying concept. Especially in the ancient world, not all the dead people you'd known would be friends. Plus, you know, incoming winter. All Hallows Eve I know less about.
Now there are lots of reasons people enjoy halloween. It's an excuse to get drunk, an opportunity to indulge in your morbid side with gory costumes and horror movies and an opportunity to come up with cool costumes. For little kids, well, candy. For the ones handing out candy, there's a sense of community and charity. Some people get really in to making their house spooky too. Some people like scaring the kids, some just like pulling pranks and it's one of the few days that is socially kind of normal.
In short, it's FUN. Getting candy is fun. Wearing costumes is fun. The people handing out candy seem to enjoy it. I remember when a friend of mine dressed up as the crow (at ten years old) and one guy had to get all his friends to the door to see it. Getting drunk is fun. Watching horror movies is fun (for other people at least).
If you don't want trick or treaters just turn your front lights off, maybe a sign but I've rarely found it necessary. Trust me, they'll be able to tell. I remember it well, they spend 2-3 HOURS traipsing from house to house, they learn to recognize when it's not worth climbing up to the door. If part of your Australian self-identity is not celebrating halloween, then don't. No need to rain on someone else's parade, especially a dumb kid's.