Is this a novel idea in Australia? They're pretty common around here (PNW US). I first came across the idea about 10 years ago when a friend built a catio for their cats. I built a one for Ozzy this summer. We don't let our cats outside unattended, but he was constantly pawing at the patio door and meowing. Any time someone went out there, he would try to squeeze through. We tried a harness and leash but he absolutely hates it and just lies on his belly until we take it off. So I built a little enclosure up against a window in our TV room. It doesn't have a large footprint but it's about 7 feet tall and has platforms on multiple levels. He has access through a pet door I mounted in a chunk of plywood that fits in the window. He loves it. Spends hours in there nearly every day. Our other cat stuck her head out the door once for about 5 minutes, and has shown zero interest since. 🤣

I built mine from scratch materials, but there are lots of prefab kits available these days.