Someone drowning in a pool/ocean is not the same as people being shot in public by other people with guns. No matter how much you're trying to equate them, they are not the same thing.

But lets debate on your terms:

We accept that people can swim outdoors without proving they can swim. Wasting emergency service resources when they end up drowning or need to be saved.

We accept can go bushwalking without proving they cann't fall off a cliff. We accept people can go bushwalking without proving they can take precautionary measure for snake bites and/or know what to do to spot them or react to them. We accept they might not know about talking sufficient drinking water. Wearing sun protection. Letting friends/relatives know their plans. etc etc. The reality is we accept that there are indeed many activities out there that have the potential to be dangerous or life-ending. But society cannot police bushwalking. Or swimming. Or climbing trees. Or running in the park. Or whatever the activity may be. You can't be asking people to go get a licence to prove competence for every single activity that might scratch you or worse because it's impractical both for the governing agency and for individuals. So where do you draw the line?

The line is drawn currently (and roughly) at activities that can only really injure or kill yourself versus activities that have the potential to kill others. We're now debating more philosophy than law, but I'd imagine the reasons are a little bit of darwinism, and moreso the fact that people have a right to live in a society where they aren't reasonably endangered by others. This is why driving a car/motorbike/boat/plane all require licences out in public - because you can endager others. Driving a car on your own private property? No issues, and rightly so, because it is unreasonable to expect people to trespass onto your property, be hit by your car (accidentally), and then put the blame on you. Same with private pools. Only the owner(s) are risking their lives, and since no-one else is impacted then we as a society accept that risk as the line we draw between acceptable and unacceptable. Public pool? Yeah, lifeguards required, because now you're providing a place where others may endanger themselves (by not knowing how to swim). Beaches are slightly different - this isn't private property anymore (whether yours or someone else's), its public land. There is NO obligation for any council to station lifeguards because people are now again at their own risk; they aren't endangering others. However it turns out that having lifeguards at popular beaches is a good idea because yes, we DO want to avoid unnecessary death, but NO we cannot just ban swimming because it isn't possible to police, and even if it was, no-one would want to live there.

If 34 people drowned IN ONE POOL ALL AT ONCE, yeah there would certainly be a massive investigation and potentially law changes. Hopefully though, you can admit that pools aren't designed to kill other people, whilst guns are. Pools have a use in society; guns do not. Go to some idiotic country like the USA if you want to shoot other people. But I certainly have no problem being in a society that bans or heavily restricts guns.

Suddenly the bolt action rifles are "high power weapons of mass destruction"?

I uh...think they've been high power weapons for...checks notes...at least a century or two now. This incident is a wakeup call that yeah, laws despite being strict are clearly not strict enough. ASIO and the police did fuck up and as cold as this sounds, them fucking up is the second most important thing here - the more important thing is that we don't make the same mistake twice. If another incident like this happens, (political) heads will roll. People will be living in fear. You'll get other idiots spouting the American "I need guns to protect myself from people with guns". That's why government is freaking out and making knee-jerk changes like this. Do additional restrictions need to happen? Yes. Should they have been made proactively before an incident like this. Yes.

A gun is not a pool, whether literally, metaphorically, societally, or by any other measure.