Geoff I don't know any Sydney suppliers. I had a quick look at the Action Aluminium (Vic & Qld only) and 150mm ( 3mm wall thickness) seemed to be the largest diameter I could find.
I've stuck in a link to their catalog in case that helps with background info (grades of aluminium, idea's on weight etc).
http://www.actionaluminium.com.au/AA2008Catalogue.pdf
Bob