You are officially divorced once your Certificate of Divorce is issued by the court. If you change your mind about issues like division of property, pensions, and debts, they will rarely be able to be changed once the divorce is finalized. It is so important to get legal advice beforehand!
Issues like parenting arrangements and child support can be changed in the future, but you would need to file a ‘variation application’ and show that there is an important or substantial (material) change in circumstances.
Issues like spousal support may or may not be able to be changed in the future. It will depend on your situation and the wording in your Corollary Relief Order.