There is also the effect of rich people needing a place to stash money and not wanting to bother with tennants

Housing activists in Melbourne have been campaigning about that for years now

I did not believe that was true until I stumbled on it happening. A rich lawyer, a salary far too big to spend, so they collected (empty) houses. They did not care about cash flow

A tax on empty houses in areas with accommodation shortage at worse cannot hurt and at best could free up a lot of accommodation