The Australian National Flag today has a blue background: the Union Jack, the Commonwealth Star and the Southern Cross.
The Union Jack in the upper left corner (or canton) acknowledges Australia's origin as a British settlement.
The Southern Cross, positioned on the right hand side of the flag, is a constellation visible from all of Australia's states and territories, and which was vital to early ships' navigation, and which represents Australia's position in the Southern Hemisphere.
In the bottom left hand corner is the Federation Star, also known as the Commonwealth Star, a seven pointed star which represents the union of the six colonies at federation. There is a seventh point to represent Australia's many territories (mainland and offshore).
The Australian National Flag, a defaced British blue ensign, was first flown in Canberra in 1901.