Luck plays a huge part, environmental factors, density etc.
Adelaide there was some pure luck last time with a doctor making someone get a test early and happened to nail it early.
Northern beaches - a lot of affluent people who drive everywhere and live in big houses, similar to how Victoria were able to handle it at the start of Covid when it was centred in Toorak.
Compare that to last years long Melb lockdown, that was West of Melbourne, meatpackers, state housing etc.
This time around, slow off the jump because the person had already been through quarantine in Adelaide, a strain that's significantly faster than previous and you're talking exposure sites at MCG, Marvel and Chapel St. That's a nightmare. They have already contact traced over 10,000 people.
Re Perth, at least in my experience, is just about the most dead city in the country. Density is about non-existent and you can walk through the main street and safely social distance. As far as infectious diseases go, that's a pretty good thing.
I'd have more fear in Adelaide where the city is far busier, albeit again, no where near as busy as Melbourne or Sydney.