The box jellyfish are a problem, but only in the tropics and only when they're in season. Predators take most of them out but by the end of the season they've built up a deadly supply of venom. Not a nice way to die.
I've heard about those but don't know much about them.
I once waded the length of Townsville Beach in the height of the jellyfish season. I'd only just arrived from away down south and completely forgot about them. It's better to be lucky than smart.
I cannot imagine the terror of being stung and knowing you have to get to a hospital before it becomes fatal. Yikes! I don't think I could even put my feet in waters that had danger like that.
November can be hot but isn't too bad. January/February are the bad months. Apart from that, you're right. The nasties don't trouble the locals -- we're pretty much used to them -- but they're a source of worry for visitors.
That is an olive python, although some reports say it's a different species. But everybody agrees that the prey is a walleroo, not a cow as you probably assumed. A walleroo is bigger than a wallaby but smaller than a kangaroo. There are many species, but they tend to be about two to three feet tall and weigh 20 to 30 pounds or so. So that is a very big snake, but not ridiculously big.