Firstly, population density of the whole country is irrelevant. Population density of urban centres (e.g. Sydney + Melb > 10M pop.), is comparable if not greater than many cities in the US. We are largely self-sufficient for most food stuffs. I assume most of the US is too (I doubt that closing your borders would lead to empty food shelves). But we are entirely not self-sufficient for just about anything that's manufactured (the bulk of which comes from Asia, mainly China). We shut our borders to people (mostly) not goods. We also had governments that were willing to make politically very unpopular decisions based on the advice they were getting at the time. And the more the outcomes diverged between what we were seeing and what the rest of the world was experiencing, it became less politically damaging to make those kinds of calls (even more so because there was largely bipartisan agreement on the macro decisions even if details varied across states).
I live in Melbourne and went through 2 multi-month lockdowns. The first was 3 months, the second was 5 months. It may be comforting to think that it was "trivial" for us to control the spread of COVID, but I assure you it didn't fucking feel trivial 5 months into the 2nd lockdown.
> but I assure you it didn't fucking feel trivial 5 months into the 2nd lockdown.
I wrote that it is trivial to prevent people entering and leaving AUS/NZ, as you also stated in your comment "We shut our borders to people (mostly) not goods."
I did NOT write it was trivial "locking down" cities. Europe is under going similar lockdowns, so Australia is far from alone. According to the press, there are 30-40,000 of your fellow citizens that still have not been able to return home 12 months after all this started, precisely for the reasons above.
> population density of the whole country is irrelevant.
It's completely relevant. Even more so that if you live in Perth, you don't just jump in a car and drive to, say, Sydney, whereas in Europe you can drive from London to Birmingham in a couple of hours, or Paris to Brussels, and so on. Australian towns and cities are very widely spread compared to Europe and the USA. Which is reflected in the speed of transmission (and lack of) across the country.
You underestimate the amount of interstate traffic here, especially on the east coast. The real reason we squelched transmission was the highly unpopular but rigorously enforced state border closures. That is a political and economic choice not a reflection of geography. And it was done so, by state governments of both political persuasions, despite the barrage of constant criticism from the Murdoch press.
I live in Melbourne and went through 2 multi-month lockdowns. The first was 3 months, the second was 5 months. It may be comforting to think that it was "trivial" for us to control the spread of COVID, but I assure you it didn't fucking feel trivial 5 months into the 2nd lockdown.