Context-verbal family agreement to pay a loan for someone. Is it a contract? If this is dragged into court via a family law matter is it void?

1. Paying a student loan for my son. I agreed to pay monthly instalments of $500. 2. Have separated. Wife is now taking me to court and also dragging in this "loan" agreement as a vexatious 3rd party, even though she is partially liable herself for repayments accorning to my son's original admissions. 3. Is this verbal contract void given he has gone outside our original offer/acceptance/ageement terms, going to her as a vexatious 3rd party who threatens to intercede on a civil law level via her family law claim? 4. Are family agreements not enforceable? This is in Australia.