Australian National Botanic Gardens
The Union Jack (also known as the Union Flag) in the upper hoist quadrant or first quarter of the Australian flag denotes Australia's historical links with Britain.
The Union Jack itself is a composite flag symbolizing the union of the historical components of the United Kingdom. It is composed of intersecting and overlayed red and white vertical and diagonal crosses on blue and white backgrounds:
England - vertical red cross on a white field - dates from the time of Crusades and the decoration of the tunics covering the chain mail of the crusaders.
Scotland - Also known as the Saltire. Diagonal white cross on a dark blue field (this colour was adopted for the general background of the Union Flag) - origin obscure, and probably nothing to do with the apostle Andrew. The reason for the white surround the the red Cross of St George is that it an heraldic taboo to place red directly on blue.
Ireland - diagonal red cross on a white background - nothing to do with St Patrick as he was not a martyr and has thus never been associated with a cross - added to the Union Flag in 1801 in such a way that neither the red or white diagonals are seen to be superior (on the hoist side the white is superior, on the fly side the red is superior.
The first Union Jack was the Cross of St George combined with the Cross of St Andrew to for the Union Flag in 1606. The flag raised by Govenor Philip on the first Australian Day, 26 January 1788, was this version of the Union Jack.
The Union Jack appears on a number of other national and state flags of countries that have had an historical connection with Great Britain.