If you want to work on commercial vessel, you must hold an appropriate Certificate of Competency under the National System for Domestic Commercial Vessel Safety.
In this section...
By law, any person working as a certified crew member on a commercial vessel in Australia must hold the relevant Certificate of Competency. This ensures that people operating commercial vessels are competent and medically fit for their assigned tasks and areas of operation.
You need to meet various qualification and experience standards in order to apply for a Certificate of Competency.