I can see why the bookies want to be careful around Australia - not necessarily because we're a shoe-in for the win, but rather because we'll be such an unpredictable factor, the dark horse of all dark horses. Depending on how our participation is received we could be anywhere from first to last, and assuming our song is slightly different from the usual Eurovision fare there'll be even less for them to go by when making odds. I think they're just safeguarding against a situation where they put us at average odds (40/1), we win through a bunch of unpredictable factors, and they have to make a huge payout. The Western bias in those sites is probably a part of it too - with perhaps the exception of last year, the odds on the UK and Irish entries have been laughably short (I remember Englebert and Bonnie both being somewhere in the top 10).
That said, I don't think an Australian win is entirely unlikely, due largely to the unpredictability I mentioned.