Hague convention on child abduction would apply as Australia is a signatory. The right of the child to have contact with both parents is fixed. One parent can only be removed from parental rights by court order, therefore technically both parents have to consent. Does not apply in Cambodia as Cambodia is not fully in that treaty at the moment. The airline should have asked for parental consent at checkin. When we flew with Qatar they did, and also Bangkok airways. Air Asia did not. Airlines are sloppy if you are travelling to the country of the childs citizenship because the child will not normally be rejected. If the child had an Australian passport and was being taken to France they may have checked.

the baby had to have had an australian passport.

with the scant information about this im doubtful if it happened the way described.