Laptops weigh about 2kg-2.5kg (including battery and charger) if they've been made in the last 10 years or so. To get to the weights you are talking about requires buying a 20" monstrosity such as this:
http://archive.laptopmag.com/Feature...ook.htm?Page=1
People aren't allowed to just take 10, 20, or even 50kg of carry-on luggage "as long as it fits into the overhead compartment" because those compartments have safety requirements. If there are 300 kg of lead in that overhead compartment and the plane encounters turbulence, then I think the compartment is liable to come crashing down on passenger's heads.
Lastly, why do you keep talking about wearing a "goat"? Goats look like this:
http://images.google.com/images?q=goat and are not suitable for wearing whatsoever.
Enough of this rambling thread! All of the information you need is on the QANTAS website (what size/weight luggage you can take onboard)