What age can a child in adelaide decide which parent they want to live with

Only the courts can decide. There may be some input from the child if they are older but in the end only the courts have that right.

First its not a good idea to piggyback your question on someone else's. This can lead to confusion. You should start a new thread. So I've moved your question to its own thread.

This question has been asked and answered many times. The answer is never. A child cannot make this decision. Only a court can make the decision as to which parent has custody. Some areas do have guidelines as to how much weight to put on the child's preference as they get older, but the decision is still up to a judge. Not sure where Adelaide is to be able to check applicable law.

If the child wants to change, then the NCP needs to go before the court that has jurisdiction and petition for a modification of the custody order.