Hi,
If you are having trouble finding dextrose and have had no luck in your local Coles, Woolies, Kmart or Big W and you don't have a beer brewing shop nearby I have done a quick Google and come up with http://www.grainandgrape.com.au/index.php
These guys sell dextrose and will ship (shipping costs calculated at checkout so not sure what they would be).
Worth checking out if you are struggling to find dextrose anywhere local.
Hope this helps,
Nicole