If Centrelink has made a decision which affects you and you don't agree with it you can challenge it by making an appeal. An appeal is a formal request to Centrelink for a review of the decision to determine whether it was correctly made. Keep in mind that complaining about something is not the same as requesting an appeal or a review of it.

If you want to appeal you should do it as soon as possible, at least within 3 months of Centrelink's decision, so that you can receive any back pay which you may be entitled to if you are successful.