It depends where you are starting from - which you haven't really said - welcome to FT, please note, if putting questions like this, more information is helpful than less.... Basically to buy a BA ticket for this routing starting at Heathrow (BA's home base) is (from a European point of view) one of the most expensive ways to do this. However, by routing (for example) Milan - Heathrow..... - Australia, the ticket may be cheaper than flying direct from Heathrow - but you would have to start your journey at Milan. if you look at the sticky at the top of the forum, there is a lot more detail about this kind of thing in there, and that should answer most of your questions - in particular this portion of it http://www.flyertalk.com/forum/showpost.php?p=3353982