Anything american or european made will likely be cheaper in London anything Chinese/Japanese/Taiwanese will be likely (though not necessarily) be cheaper in Sydney. For certain you will pay a lot more for imported stuff in australia than you will in London. since shipping is free buy/build before you go and make sure you are comfortable with the bike that way you will need to order minimal product long distance. Enjoy Sydney it was the one place I seriously considered leaving the USA for when I retired from the Navy
Really? Everything made in Taiwan should be cheaper in Sydney(that's pretty much every bike on the market right now!)?? Nope. Prices here are insane. Buy in London. Most people here order from UK mailorders to save money. There are some good bike shops, and theres some that sell some decent fixies, but you can get more for your money in London. Or if you come down here and buy you should swing by Hell on Wheels and check out what they have in the shop.
I didn't say it would be...I do remember bike parts and skateboard parts being rather expensive when I looked in shops in Sydney, the one thing I remember being real cheap was camping and climbing equipment