Check out the TAFE courses. I recently did the Coxswain course there, enrolled for the Master 4 but decided against it as it was focused on big ship driving and I wanted to be more focused on a Yacht.
Once you have the Certificate you then have to approach Maritime NSW or the equivalent in your state. They will want proof of seatime. For the Coxswain entry level they will accept recreational hours but above that it has to be on a boat in survey ie under a professional skipper.
The TAFE courses are long and frustrating, not all the kids on it are keen students. Coxswain was 7 weeks I think but you can do it with a commercial school in much less time.