All of the above are supposed to get bigger and bigger as you get further north. Well except for leeches, we have them big enough right here. Wilson's Prom is a good place to collect them. If you get them either a lighter or salt will loosen them and you can get them out, they don't really drink much.
Ticks are more dangerous and some are poisonous, you also have to 'unscrew' them to remove them apparently- again they get more prevelant and bigger the more north you go apparently. I've never seen one but a nephew got one in Healesville once.
Scorpions can be lethal apparently, I've seen them here, we had them in Healesville and in Montrose in our house, but only tiny ones. I don't think those ones had enough in them to do much to us but with young babies around at the time it wasn't something I was prepared to risk.
If you're in leech infested areas especially when it's wet cover up completely- socks over pants, scarves over necks etc. Then do spot checks whenever you stop walking and have your lighter handy.
As for scorpions, you're about as likely to find one of them as a spider.. just watch out under tree bark and similar hidey hole spots like that, maybe when collecting firewood you might be unlucky enough to find one. I've only seen them when I've gone looking myself and they're pretty easy to avoid then.