From my perspective, there are a few ways to handle this situation. I'm not a professional by any means, but hopefully these thoughts may help you.
You and your girlfriend have been going down the right path with regards to contacting the police, and should continue gathering evidence against the guy. As /u/smexy_gorilla suggested, investing in a cheap camera for your home and documenting all the text and call exchanges might help build a case against him. I don't know whether or not it is even illegal to stalk a person in Australia, but this should at least allow you to continue renewing restraining orders.
If this guy really is as big and aggressive as you describe him to be, I would suggest that you and/or your girlfriend attend some classes on weapons training. I saw another user suggested "self-defense classes to build up your confidence", but that's the type of thing that gets people killed. A woman weighing 140 pounds that knows a martial art like judo or karate will still get mercilessly beaten by somebody twice their size and being an amateur in it won't serve as an effective deterrent. Mastering the use of a combat knife will end up being much more effective in case she is assaulted again, as you can effectively stop the threat with just one well placed strike in a self-defense situation. This is the best way to counteract a severe deficit in strength and size.
If you really have the time to invest in this, you might want to try counter-stalking. Find out as much information as you can about the guy and use it against to soften his resolve. I believe that he thinks he holds the power in this situation since he can act as a shadow and be menacing towards your girlfriend. If he sees that he can also be stalked and he is no longer the one in the power role, he might end his reign of terror. This of course would be the biggest nuisance of the three options.
I would suggest blocking all forms of communication from him, but that might make him more aggressive in a physical sense, so it may be better to just continue receiving messages from him to document.
Hopefully this helps you out a bit. Again, I'm not a professional in any way, shape, or form. These are just my thoughts.