You will require this registration if you operate a ship in Australian waters for the purpose of international trade.
The purpose of the International Register is to provide an alternative to registering overseas for Australian companies that own or operate ships overseas. Inclusion on the International Register is optional for owners and operators that are predominantly engaged in international trading.
If your ship is over 24 metres in tonnage length you must fly the Australian Red Ensign. All other registered ships may fly either the Australian National Flag or the Red Ensign.