If you do come here, even for a visit, you may want to go to the local department of social and health services and see if there are any child protective report or cases pending that involve your son.
More than one dad has lost children to the DSHS system when they live in another country.
There is a very high profile case in Florida right now where a woman's children were taken into protective custody and they adopted out the kids. The Cuban father has had a hellacious fight to try and get his son back.
DSHS decided since he had not seen the child for a while that he was by default an unfit parent and decided they had the right to adopt the child out.
So, he came here on a visitor visa and never came back. Have you talked to your local police to see if it could be considered kidnapping in your country and you could get your child back through the hague convention?
Is there a formal custody agreement in place at this moment that she is in violation of?
It takes a lot of time and money to fight an international child custody or kidnapping issue. If you are committed you may want to talke to the local police, then a local attorney, and will most likely have to travel to CA to see what is going on and gather evidence.
Depending on if there is already a formal custody arrangement may determine where jurisdiction is. If you can keep in Aussie at all you will want to get on it and file something there and have her served before she ends up filing something in CA.
Good luck!