Registered commercial ships over 24 metres in tonnage length must fly the Australian Red Ensign. All other registered ships have the choice of flying either the Australian National Flag or the Red Ensign.
An unregistered Australian owned ship can be issued with a certificate entitling it to fly either flag. Some ships are allowed to fly other flags in Australian waters only. These include: a State or Territory flag, a flag or ensign authorised by warrant under the Flags Act 1953, and the British Blue Ensign if the owner intending to fly it has a warrant to do so valid under British law.