Greece is interesting. The EU/Merkel will always say that they are prepared for Greece to leave the Eurozone, right up to the moment they change their mind.
Greece is a small part of the European economy but nobody really knows the impact of Greece defaulting on their euro denominated debts(which will have to happen once they leave the euro). European banks are still not in great shape(but who they have hedged their Greek exposure to nobody knows - US banks? Asian hedge funds? Australian super funds? Nobody knows. Probably all over)
However, the bigger issue is how the people of Italy & Spain react - if Greece leaves the euro, and the world doesn't come to an end, their will be huge pressures on left wing parties in Italy and Spain to "do a Greece". Leave the euro, convert all the county's debts to whatever new currencies those countries will be using(ie lira) and life continues with a hugely reduced debt burden, allowing those economies to grow.(Italy's economy is smaller than it was in 2000!)
Yes the new currencies will be significantly weaker than the euro, but that is a small price to pay.
My personal view is the EU/Merkel will continue to say Greece leaving the Euro isnt important, as a negotiating ploy. Once they realize Greece is serious about heading for the exit they will quickly try and change their stance. It is likely to be problematic though, if they give Greece too much Italy and Spain will want the same, if the EU dont give Greece enough they will leave the euro, and Italy and Spain will be likely to follow.
Ultimately in democracy's you cant hold a currency union together if is causes severe recessions/depressions in the countries that make up the union, both Greece and Italy have suffered worse than they did during the Great Depression. Spain isnt much better. Why should those people want to stay when it causes them and their families so much hardship? So they can remain "good Europeans?" Not likely!
IMO it is almost certain Greece will leave, and greater than 50/50 that both Italy and Spain will leave as well.
Uncharted waters, historically when there are major developments like this the AUD doesn't do very well. Look at Sep/Oct 2008 but its impossible to predict.