OP, I have been the parent of a teen in an extremely abusive relationship and a lot of what you have described is horribly familiar.
Could you take him away for a few days? I know she will still be there when you get back but I remember those first few days as the ones in which my DD kept waivering about whether it she had done the right thing by ending the relationship. She gradually began to regain her perspective on the situation as that time progressed.
Also, is it possible to send him to a different school; preferably a boys' school so she can't follow?
She will probably fight tooth and nail to get him back, whether the police are involved or not.
Coercive controlling behaviour is illegal in the UK now. I know it's a long shot but it's worth finding out if the same applies in Australia. You may need to help the police to recognise that this is what is going on.
I think you need to clear your house of anything your son can harm himself with. Medicine, belts, knives etc.
This is a lot more difficult than it might seem and would probably just alienate your DS more if he became aware of it. I would just make sure any large quantities of medication were hard for him to get to on the spur of the moment.
FWIW, I think you have done the right thing at every stage throughout this whole sorry saga. My DD is very clear that what rescued her was my consistent efforts to maintain a relationship with her throughout. You, too, have prioritised the relationship over your own emotions.
You've now been pushed into a corner. When I was in a similar position and had to put my foot down, she defended him at first and then suddenly broke down and sobbed, agreeing that she knew she had to end it. We had an awful few weeks as the full extent of the abuse became apparent but it's behind us now.
However, I'm not sure she would have managed to stay away from him, had they been in the same place every day after the relationship ended - hence my question about moving school. I'm sure this won't be a good time to move in terms of qualifications and the school is likely to play the whole thing down and prioritise results but this is an extremely dangerous time for your DS. Please don't underestimate the power this whole family has to draw him back into their lives, isolate him from you and destroy him.
Words can't express how much I hope you are successful in getting this girl out of his life. The power that coercive control has is phenomenal and far too few people recognise it. I hope you're lucky and the police officers who you turn to are well trained and have the power to help.