Australian International Maritime Flag

Noosa River, Queensland, on board the Noosa Ferry..

The most senior position for a flag on a vessel is reserved for the Ensign - this is as close to the stern of the vessel as possible. The Ensign shows the country of registry of the vessel and indicates its nationality.

The Australian Red Ensign is a predominantly red version of the Australian National Flag, using the same shade of red as the Cross of Saint George which is part of the Union Jack present in the canton.

It has three elements, the Union Jack in the upper left corner (or canton), the white Commonwealth Star underneath it and the Southern Cross on the fly (or right hand side) of the flag in white.

The British Red Ensign is used by Merchant ships and private vessels, and in several Commonwealth countries (including Australia) the Red Ensign has the addition of emblems/flags of that country.

Added 3rd November, interesting history from Wikipedia... The Red Ensign or "Red Duster" is the civil ensign of the United Kingdom of Great Britain and Northern Ireland. It is one of the British ensigns, and it is used either plain or defaced with a badge or other emblem, mostly in the right half.

It is the flag flown by British merchant or passenger ships since 1707. Prior to 1707, an English red ensign and a Scottish red ensign were flown by the English Royal Navy and the Royal Scots Navy, respectively. The precise date of the first appearance of these earlier red ensigns is not known, but surviving payment receipts indicate that the English navy was paying to have such flags sewn in the 1620s.