A Deck Certificate of Competency is required in order to operate as master or mate of a domestic commercial vessel. There are various prerequisites, courses and sea time requirements to be satisfied depending on vessel size and area of its operation.
Vessels operated commercially must be under survey unless exempted under the Northern Territory Marine Act. Depending on the size of the vessel and its propulsion machinery, a certified marine engineer may be required.
Further information can be found at http://www.amsa.gov.au/vessels/national-system/seafarers/index.asp