As long as you are in Australia
, you need to comply with local state licencing laws (they vary - in NT, you don't need any form of registration
To leave the country, you will need Australian registration
for the boat. You will not need any form of licence once you depart.
Edit: This assumes that you are starting from Australia and purchasing
your boat there. If you buy it overseas and start from there, you have other options for what country you register the boat in. But the bottom line is that there is no internationally required form of licence or qualification to sail a pleasure boat.