Other than 2 engine operation units of competency the main difference between a Coxswain NC1 and the NC2 and NC3 is the requirement to complete the Coastal Navigation unit of Competency MARH013. The good news is that this is a low cost learning component and can be completed completely online.
When you have completed the marine radio licence course you will then be able to apply for your Marine Qualification such as Coxswain or Master <24 metres etc. with the Australian Maritime Safety Authority (AMSA).
Go to the AMSA website using the link above and complete and submit Form 426.