Basically no Australian wildlife can or will seriously hurt you. There are some snakes which if you try really hard you might run into (for reference I've had 2 encounters with dangerous snakes in my life), a couple of spiders one of which is extremely common but not at all aggressive and does not roam, kangaroos if you hit them with your car you will be fucked up, emus and cassowaries if you go where they are and try really hard to make them angry, crocs up north if you approach them can be dangerous, box jellies up north will kill you if you swim when they're about, dingoes in the centre are wild dogs so if they're hungry they could harm you.

you can go bush walking with no plan and just some water. Wander off the track all day, and your biggest danger is getting lost. Avoid that and it's dehydration or falls. If you stick your hands into dark crevices and overturn bark and stuff I guess spiders and snakes?