If your ex husband has not yet removed your child from Australia
Apply to the Family Court for an injunction to prevent your ex husband from taking your child overseas. Other steps: place your child on the airport watch list, alert the Passport Office to prevent them from issuing your child a passport (if no passport is already issued). The Passport Office will not issue a child a passport without the consent of both parents. If your ex husband wants to apply for a passport without your consent, he will need a court order. Therefore, you can apply to the court to prevent your ex husband from obtaining one.
If your ex husband has already taken your child overseas
If you have a genuine fear that the father will abduct the child to a foreign country, then you can apply to the court for an urgent parenting order that restricts travel with the child and places them on the airport watch list. Alternatively, the court might impose a bond that facilitates the travel but places funds from the other parent into a trust account to be used to cover legal costs if proceedings to return the child have to commence.
I believe the form is on the Family Coirt website.