If you wish to build a garage, shed or carport you will require a building permit if the structure is larger than 10m². The permit will not only cover the design and materials used in your project, but there may be regulations as to where your structure is situated on your block.
You can build a garage, shed or carport as an owner-builder or you can engage a registered builder or a company which specialises in garage/shed construction.
Irrespective of who undertakes the construction, it is your responsibility to ensure that a building permit has been applied for and granted. Do not assume that your contractor has done this for you.