Speaking as an Aussie, I can tell you that it's much easier to emigrate Down Under if you're a skilled worker. Consequently, you guys should be fine - particularly since your gf's skills are in demand (Australia is currently suffering from a shortage of doctors).
Details of the process for skilled migrants can be found
here. For information specifically relating to doctors and nurses, click
here. For a useful overview of life in Australia (including details of the capital cities) click
here. For anything else, just ask me.
Now a few words of advice...
Australia is not for everyone. If you have no compelling reason to go... don't.
Write a list of the things you really want out of life. If you already have them in the UK (or you're already on the way to getting them) then why look elsewhere? Australia won't necessarily change your life for the better; it's not a magic wheel that spins straw into gold.
Like most places, Australia is a land of swings and roundabouts. You have to trade them off against each other. Some things are cheaper - a lot cheaper - than the UK. Other things are more expensive - a lot more expensive - than the UK. Find out what they are, and compare them. Try to estimate their impact on your family budget. If you have the opportunity to visit Australia on holiday, take it. Holiday life is not the same as "real" everyday life, but it will give you an invaluable insight into the country and its people. I recommend 1-2 months; make sure you visit more than one state.
Consider your relationships with family and friends. Are you so close that you can't live without each other? Are you fairly independent? Could you cope effectively without your usual support network for months on end? Do you make friends easily, or are you happy to have "quality over quantity"?
Do you enjoy regular overseas holidays on the continent? Bear in mind that these will be almost unaffordable when you're flying from the southern hemisphere. Driving to Paris is great fun, but the Chunnel doesn't quite reach Australia. If you're the sort of person who would miss the benefits of proximity to Europe, think carefully before making a decision.
Above all, don't move to Australia "just to see what it's like". If that's your primary motivation, take a holiday instead - and make it a long one.