Given that the reduction in cases fitted closely with expected modelling, and only began after stage 4 lockdown was enacted, it's a logical conclusion that at least some measures of the lockdown were effective at containing the spread.
That's not to say that none of your suggested reasons weren't a factor, or that every single one of the lockdown measures were effective. But on balance, the lockdown and other governmental measures were effective.
Montreal is a city of similar size, wealth, and demographics to Melbourne, and has 88k confirmed cases, compared to Melbourne's 20k. That kind of difference is more than discrepancies in testing methodology and case definitions. Incidentally, ten days ago the city went into lockdown (although less strict that Melbourne), so if in a few weeks there's a subsequent drop in cases, I think that it will be pretty safe to make the conclusion that lockdowns work.
Victorians already had antibodies - absolutely no evidence of this and plenty of evidence to the contrary, if we had antibodies the second wave wouldn't have happened.
we could just be a healthier - Healthy people still get covid.
we don't have subways - Plenty of packed commuter trains and trams, having them underground makes no difference.
Aside from all that, the case numbers correlate extremely well with when the lockdowns started and when their severity increased (https://chrisbillington.net/COVID_VIC.html), there are zero other rational explanations.
Here is an interesting paper too -> http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.552... (Co-authored by Donald Henderson who was credited with ridding the world of smallpox) (talks about the pros and mostly cons of lockdowns)
Edit: Just adding a link to a paper that suggests T-Cell immunity from previous beta corona viruses might be helping -> https://www.nature.com/articles/s41586-020-2550-z
Lockdowns may work. They may not. Nobody has a clue, despite all the studies, due to so many confounding factors and ways and means of measurement.
It could of just been;
- seasonality
- Victorians already had antibodies from other strains from years prior
- our population may have not be as old as other places
- we could just be a healthier society in general
- we don't have subways
etc
But that is a rabbit hole discussion I won't have the time to go into;
I think any information about Covid is just noise at this point, everyone (including me) just comes up with some shallow statements about what is going on with just a few data points.
Every region of the world reports numbers on everything differently;
- What test kits do you use, and what are their +/- rates
- How do you report somebody died? (assumed covid, had covid, died from covid)
- When does an active case become inactive?
- How long do you think it incubates for?
etc
There is probably another 100+ important questions to ask.
And then ask all those questions to different states/countries and all the answers are different.
Yet we compare countries all the time.
HIV/AIDS had a similar problem, you could have AID's in America, fly to Australia and we would of reported you as maybe not having AID's.
"Even AIDS itself has a different definition depending on where you’re standing. In the U.S., AIDS is someone whose little virus-fighters, their CD4 cells, are below 200 per cubic mm of blood (normal is 500-1,500) or who is already showing symptoms. The WHO definition is anyone with a CD4 count below 350. In most European countries, you need to have an AIDS-related infection to be diagnosed with AIDS."
https://newrepublic.com/article/117789/aids-data-country-why...
Pretty good article, more or less my stance on Covid at the moment.
So when we say the US had 400k deaths and Australia had 3k, I am extremely sceptical that we are reporting things in the same way at all.
Edit: Co-authored paper by Donald Henderson who was credited with ridding the world of smallpox talking about the effectiveness of lock downs -> http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.552...