Before commencing your project it is important to familiarise yourself with the building approval process.
The role of a certifier is two-fold;
1) To provide assessment of the documents you supply (from the checklist); in correlation to the collection of prescribed government determined regulations. Basically the "what you wish to build" matches the "what you are allowed to build". If they match then a Construction/Complying Development Certificate approval is issued.
2) To check each mandatory stage of the building work is consistent with the prescribed regulations, the "what/how you are allowed to build" matches the "how you are actually building it".