Air-con is for sissies! Brissy isnt that hot. Apart from early august to late may....
I'm not really a laptop expert, all the advice I can give you is to avoid 'switchable graphics' or 'optimus', and try to avoid some of the more dodgy brands. I'd probably get a lenovo, they always impress me.
Really nice list of lpatops with excelent linux support here-
Its even a .au site. "All prices include GST. Delivery available to most locations in Australia".
I've never bought from them, as I build my own desktops and dont really use laptops. If the prices aren't silly, I'd consider buying from them, its a nice list and supporting companies that support linux is a good idea IMO.