Some of the fast-food chains sell super-sized Quarter Pounders, which contain about 220g of fat and 11,500kJ each, on request.
The "double pounder" is the equivalent of more than three days' worth of fat for a grown man - or almost five Pizza Hut deep-pan Hawaiian pizzas or 45 Tim Tams.
At about $18, it is not listed on the menu, nor under nutritional information.
It has developed a cult following as a challenge for teenagers, after a video was posted online of a 17-year-old boy ploughing his way through the burger at a McDonald's restaurant.
Rival chain Hungry Jacks is also allowing customers to order mega-burgers under its Have It Your Way ordering system.
Nutrition Australia senior nutritionist Aloysa Hourigan described the double pounder as "obscene" and a "heart attack waiting to happen".