If the link fails, relevant information: Squatting is not, strictly speaking, illegal in Australia. If the doors are locked and they break it it's breaking and entering, and if the owner asks them to leave and they don't it's trespassing, but if the property looks abandoned, the doors are unlocked, and the owner hasn't shown up to say no, you can move in. And if you manage to stay there for 12 years (or 15 in Victoria), adverse possession, or squatters' rights, kicks in and you can file claim to the house.
Basically, what he's doing is completely legal because squatting laws were specifically put in place to avoid people making empty houses for profit, and rich people are upset and calling him a far-left criminal activist for telling people things they don't want them to know.