The constitution actually states that the government can't regulate political discourse. You are free to spout whatever bullshit political views you want, no one can't really stop you from doing so. The courts can only really start getting involved where there is something else involved.
Andrew Bolt can spew all the crackpot political shit he wants. When he says shit like "Indigenous people with mixed heritage are just as lazy as fully Indigenous peoples but they're not real Indigenous people so should not get government support", the courts can get involved.
In the case of Rundle Mall, there was a specific council law that stated that no one was allowed to "preach, canvass, harangue, tout for business or conduct any survey or opinion poll" without permission on roadways. This limits political discourse and the key thing to note is that the High Court did take the constitution into consideration as the government cannot regulate political discourse.
But they came to a conclusion that the council law was theoretically valid and, while it unfortunately limited political freedoms, the preachers are breaking the law and cannot limit the safe and convenient use of the roads. Which is an entirely fair result if they were truly disrupting the flow of traffic day in and day out.