This image from a NASA satellite is just amazing and shows the extent of the dust storm over eastern Australia on September 22-23 2009. The image covers 25 degrees of lattitude which works out to be 1500 nautical miles north to south, or just short of 1800 statute miles. That makes the dust storm front somewhere around 1500 miles or so. Notice the southern edge of the storm is out to sea and covered by clouds in ths image and the northern edge is far northern central Queensland. (FXBUXTVG5J62)

Click the image to get a huge resolution version from NASA.