(updated/completed - 9th October 2008)
Brisbane's Biggest Burger? Maybe. Since I haven't yet found anything else like it on the internet, maybe these are the biggest in Brisbane.
The plan originally was to construct a giant 20-40kg burger, starting small (5kg) and working my way up to whatever size was practical/possible for me to do on my own.  After four Giant Burgers, the project has now come to an end, with no practical means for me to construct one much above 10kg.
This webpage documents the project.
Forgot to mention - this burger project has now been featured on the Supersized Meals Website.
The project stages:
0 - Introduction (story behind the Giant Burger Project) - (detailed below this listing).
1 - Prototype Giant Burger (Super Whopper - 5kg) - Done Saturday 8th March 2008.
2 - Second Giant Burger (Super Aussie Burger - 6kg) - Done Friday 21st March 2008.
3 - Third Giant Burger (Super Bacon Deluxe - 9kg) - Done Thursday 29th May 2008.
3a - - Side projects (World's Hottest Burger, Cheeseburger pizza, Cheeseburger pies, deep fried Cheeseburger)
4 - Fourth Giant Burger (Super Quad Stacker Deluxe - 11kg) - Done Saturday 4th October 2008.
This whole project started out mid 2007, while in America for the Annual Transformers Convention, called BotCon, I stopped off at a Burger King (they are called Hungry Jacks in Australia) to sample some of the current items not available in Australia.
The item that caught my attention was something they call the BK Quad Stacker - four layers of meat, four layers of *American* cheese (different to our cheese), bacon and a special sauce.  This big unhealthy burger was just so American, I like to call it the 'welcome to America burger'. It was the first thing I ate while in America last year, and I've never had anything like it before. (photo of actual burger below).
For more info on the BK Quad Stacker, go here:
Discussion at work one day late in 2007 had me mention why I don't eat McDonalds (haven't bought anything from there since August 2000), which was in part due to my addiction to Cheeseburgers (I would order a dozen at a time).  Discussion on burgers went to my American Quad Stacker experience (detailed above), and so I started getting links emailed to me by a co-worker of giant burgers, most from the one restaurant in America called Dennys Beer Barrel Pub. A restaurant that actually offers as a regular menu item, a 5kg (11 pounds) burger and a 6.8kg 15pound burger!!! Denny's Burger menu.
Some of the biggest burger links sent to me (all appear to be American). (a Denny's burger) (Denny's burger) (Denny's burger),0.jpg (Denny's burger) (A 50kg burger, probably from Dennys as well)

"The Absolutely Ridiculous Burger", is being made commercially available from Mallie's Sports Bar and Grill in Detroit Michigan. 61-68kg burger for $350-400.

The only Australian big burger article I could find online was this one, claiming to be Australia's biggest Menu Burger.

Moo Moo Cafe at Mooball in northern New South Wales, sells the 500g burgers for $10.,23739,20700695-5004581,00.html
As you may notice, those big burgers were constructed in restaurants, which are equipped to prepare such a large item (the bread and meat components.  But this didn't deter me, because I was shown this 13.5kg home-made giant burger, documenting the process:
I started out by constructing a prototype burger in my own home, aiming for the 5kg mark, and then evaluating my odds of creating anything bigger than that, with or without professional help (for the bread and meat components).
A theme needed to aim for with the type of burger and the ingredients needed. As a bit of a fan of Hungry Jacks/Burger King, I decided to do a Super Whopper.
The result can be found here - Prototype Big Burger.

griffin @
(Disclaimer) This project and website have nothing to do with Hungry Jacks or Burger King in any way.  Names of their burgers have been used without permission, but these Giant Burgers are not for commercial purposes.  I'm just a fan of that particular junkfood franchise, and of the Whopper - a real man's burger (because real men don't put tartare sauce on their beef).