Then if properly defended your daughter should have good prospects of being treated very leniently, or perhaps even defending the charge if self defence can be proven, but I would certainly consider engaging a criminal lawyer to defend the charge. Regardless of the provocation, and unless she can prove that she had to throw a punch in self defence, the court will likely find she has done the wrong thing, and the courts take a very dim view of violent offences so some punitive order from the court is very likely, and in theory could be severe and can involve a maximum penalty of 20 years in theory, though what you describe would not involve anything like that kind of severity, particularly if the ex did not actually suffer a very serious injury. Nevertheless you must take the charge very seriously.
With proper representation addressing the evidence that she was herself threatened/assaulted and the provocative background to this incident, I suspect the police may be persuaded to downgrade the charge or that a court can be persuaded to impose a suspended sentence or similar non custodial sentence.
If you need help locating a suitable criminal lawyer contact the WA Law Society as they can refer you to suitable lawyers in your area.
I trust the above assists.
Good luck and please rate my answer.