It would be big news and I'm sure some people would be ranting about political attacks, but I doubt anyone would be making threats or protesting. No one actually likes politicians here, for the most part, and people aren't married to particular political parties - swing voters are pretty common. If a past PM was arrested, most people would just mutter about how much politicians suck, watch the news for the drama and entertainment, then move on.
Our most Trump-like far-right senator Pauline Hanson actually did spend time in prison for election fraud (overturned on appeal) which she labelled as a politcal witch hunt. I was only a kid at the time and I do remember it being controversial, but I don't recall any big division amongst the general public about it. Just another potentially corrupt politician.