Australia was once a colony of the British Empire and is still to this day part of the Common Wealth (England's Queen being the head of state).
I'm guessing it was put their as a reminder of their historic routes and to help in courage maintaining their ties.
The Union Flag is in the corner because it is thought locally to symbolise Australia's history as six British colonies and the principles upon which the Australian Federation is based. A more historic view sees its inclusion in the design as demonstrating loyalty to the British Empire.
Because it used to belong to the British empire. Other ex Imperial countries also have the Union Jack. e.g. New Zealand, Fiji.
The top left hand corner is the location of the union jack
The Union Jack (Union Flag) is in the upper left-hand corner of the Australia flag. "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 background." Source: http://www.anbg.gov.au/oz/flag-union.html
The Union Flag of the United Kingdom is in the top right hand corner of the flag of Australia
If you refer to the Blue, Red and White design on the upper left of the Australian flag, it is the Union Jack indicating Australia's heritage.
The Australian flag does not have red stripes. In the top right hand corner of the Australian flag is the Union Jack, the original flag of Britain, the country that colonised Australia. 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. In the bottom left hand corner is the Federation Star, a seven pointed star which represents the six colonies, with one point to represent Australia's many territories (mainland and offshore).
The Union Jack.
The Australian flag has a representation of the Southern Cross constellation, or Crux, on the right hand side. The star in the upper right corner is called Gacrux.
The Australian flag has the Union Jack which is the British flag on the top left hand corner because the British colonised Australia, and the Union Jack symbolises Australia's continuing ties with Britain. Australia has no continuing ties with the UK having legally separated in 1986 with the coming into effect of the Australia Acts on the 3rd of March 1986 . The Union flag is simply a hangover from Australias colonial era .
Cook island
The Australian flag is blue, with the red and white Union Jack on the upper left corner. Below the Union Jack is the white, seven-pointed "Federation Star" or "Commonwealth Star", denoting Australia's six states and its two mainland and numerous offshore territories. Dominating the right hand half of the flag is the constellation of the Southern Cross, consisting of five stars. See the related link below for an image,
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. 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 three main symbols of the Australian flag are the Union Jack, the Southern Cross and the Federation Star. 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. 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).