The average temperature in Brisbane now is roughly the same as the summer average across the continental US, even if some places are much hotter. I'm not saying it's hotter there than Phoenix, but it's still very close to summer weather by a lot of place's standards. And I'm not just talking about the US. The Europeans think it's going to save them too. Even in the peak of summer in London, it's nowhere near as warm as Brisbane is right now.
Here's a more authoritative person saying what I'm saying. Summer will help, but there's no reason to think it will help us much as people think it will.
Will warmer weather stop the spread of coronavirus?