We have nasty spiders and snakes, but you don't use firearms to kill either of those. Both only strike humans defensively. Our large land animals are all herbivores; kangaroo, emus and cassowaries have a very nasty kick but they'll run away in preference to attacking you. Dingoes, despite the high-profile death of Azarea Chamberlain back in 1978, are basically wild dogs, and represent little threat to people.
We also have a collection of potentially lethal acquatic species, including the Blue-Ringed Octopus, several species of jellyfish, and some sharks. Again, guns aren't a lot of use against them.
Crocodiles, which I guess you're referring to with the giant knife reference, are the one animal that will actually try to eat an adult human. They only live in the tropical north of the country, far away from the major population centres, and any that move in near the cities in those regions are killed or relocated by professional shooters.
So, no, you don't need a gun to protect yourself from the wildlife in Australia. And despite some myths, if you want a rifle or shotgun for hunting or target shooting, or need one for farming or pest control, you can get one in Australia. You just can't walk into a gun shop and buy an AR-15 or a big-calibre handgun for "self-defence" here. And, nearly 20 years after the changes to the gun laws, that remains overwhelmingly popular here.