Halloween really isn't a 'thing' in Australia. You get the occasional street where some kids do trick or treat (I believe the done thing is to put out balloons if you want to be involved), and you'd probably find a few Halloween parties at night clubs etc. In the big cities, there are sometimes some low key Halloween activities around if you look hard enough. But I wouldn't bother to bring costumes or anything unless you find something specific you'd be interested in.
The weather...
The top end of Australia (including Uluru) will be just about to get into the wet season. I would be expecting hot, humid (in Cairns, not so much Uluru), and maybe wet. Take a raincoat.
The down under part of down under will be about to get into summer. This means you can probably expect Sydney to be warm and sunny. I would generally say Melbourne will be a few degrees cooler than Sydney and should be sunny as well, but we can definitely get anything here. It's been known to be freezing cold and hailing, and it's also been known to be hot, hot, hot and sunny. And sometimes, yes, we do get a little of everything. Layers are key to surviving Melbourne weather, and it's always worth carrying an umbrella, even if it looks beautiful and sunny when you leave your hotel. The weather can change dramatically in a matter of minutes. Hobart, although the furthest south, is actually fairly temperate and not much colder than Melbourne. Plus, the weather is a lot more stable. I imagine it would be jeans, t-shirt and light sweater kind of weather mainly. If the wind picks up though, it can get very cold, so it's worth taking a few warmer items in case you do need to rug up.
I would probably pack:
* Shorts or lightweight cargo pants
* Jeans
* T-shirts (or lightweight tops)
* Long sleeve skivvy
* Lightweight sweater
* Heavier sweater/lightweight coat
* Beanie (doesn't take up much room, yet helps so much when the weather turns!)
* Raincoat
* Sun hat
* And bathers of course! (I assume you will be snorkelling or something at the reef)
Oh, plenty of sunscreen too. Remember, there's a hole in the ozone layer down here, and the sun is fierce, even when it's not that hot. Keep re-applying throughout the day to avoid looking lobster-ish. It seems counter-intuitive, but you will need to be the most careful in Tasmania, as that's closest to the hole. Seriously, I barely put sunscreen on for the whole 3 weeks I was in the US last August/September, spent most of my days outside, and the worst I got was a hint of pink at
Disneyland, which turned into a nice tan. In Australia, I'll be bright red and sore after an hour in the sun, which is always followed by peeling and turning white again lol!