This is something I've thought about a lot so humour me here
I don't find this very surprising. I'm an Australian living in London but I'm not what anyone would call a stereotypical Australian, or even just a 'typical' one. I'm of mixed race heritage (father: white, mother: filipino) and most of my friends growing up were from SE Asia. As a result my accent, culture, interests etc deviate from the Aussie norm (throw in homosexuality and it's all a big mess). So most English people don't pick up that I'm Australian on first meeting. But anyway, being the exception has given me a unique perspective on the 'Australian in London' condition.
Obviously, lots of Australians have come to London over the years and something that people have noticed is that they seem to put on the 'Australianness' extra thick. For example, they add more twang to their accent, speak more loudly and generally make it their business to let everyone (interested or otherwise) know that they come from the land down under. This is why everyone hates Australians (well hate is strong word). Add alcohol and they can be reaaallly annoying. Just go to a Walkabout and see.
I've always wondered why my fellow aussies do this and here's what I think...
Australia is a very young country with not much history or a defined sense of national identity beyond that of novelty and peculiarity (anyone remember the lawnmowers at the 2000 olympic opening ceremony?). It is also a very isolated place with the nearest country with a significantly different culture being a 6 hour flight away for most people (New Zealand doesn't count). Sure there are immigrants in Oz but their cultures still exist in the context of the general Anglo hegemony. Anyway, the point is Australians generally don't know who they are and aren't regularly faced with situations which require them.
That is until they come to London. When Australians are plopped down in the middle of different culture they suddenly become aware of the qualities which make them different from everyone else surrounding them. These points of difference are what form the basis of an Australian cultural identity. Aussieness only manifests in contrast to non-aussieness. So suddenly, these Aussies have a new toy in the form of a defined national cultural identity and they LOVE to play with it and tell everyone about it . Hence, the exaggerated Aussie accent, the 'oi oi oi' in the pubs, and general in your face behaviour. This is why Australians are getting a bad rep. We're the new Americans.
Of course this is all generalisation and speculation. There are plenty who don't follow this model but it's apparent enough to be worth thinking about anyway.
So, moral of the story, please forgive my rowdier country men and women, we're all just a bit naive when it comes to being a foreign national.