It's quite clear that during 2020, the case numbers flattened then declined once the lockdown restrictions reached a certain level. (I live in Melbourne so experienced it first hand). Other Australian cities and NZ have been able to contain outbreaks with short lockdowns, and have been able to keep the countries Covid-free, more than any other countries. There is no other plausible explanation for the suppression of the virus that was achieved in 2020.

(For what it's worth, with emergence of the delta strain, suppression measures are not working so well, and the government policies are having to adjust, which is starting to happen now in Victoria).