It's a piece of cake... grab the forms from immi.gov.au... make sure you've got all your bits and pieces together (the documentation is really straight forward - including "proof of relationship") and start preparing your home for your fiance to arrive...
Wait times vary but I think they quote about 5 months average but I believe it depends on time of year among other things...
My wife's Prospective Marriage Visa was approved in a few weeks from memory. I'd planned another trip to Ukraine at Christmas (that year) but she called me after about two and a half weeks saying the Embassy had called and her visa was approved... we both nearly died of shock.
(p.s. My wife is from Ukraine so while they may say it's no different I've known men with RU fiances who've had to wait more than 5 months... who knows.)
Oh, what I can tell you is that
the Embassy will make you proud. Very professional... very understanding and helpful - nothing at all like the stories you'll hear of other Embassies that appear to try to make it hard.
Examples:
1. My MIL still cannot believe she get's "client Satisfaction Survey" calls after she goes in to get her tourist visas and thinks it's "so strange" the Australian Embassy will call her to make sure she thought the service was good and that she was happy with the staff.
2. My wife said her interview was like having a chat with a friend... when the interview was over she said, "aren't you going to ask me some questions???" Embassy worker said, "I already have... I'm sure you'll be very happy in Australia."
3. We had a problem once (my MIL's first Tourist visa) because MIL arrived late for her interview and she didn't have much time to get it approved (wife was pregnant and tickets were booked). I sent an email and they called her the next day to arrange another interview the next day, approved on the spot and flew out a few days later.
If you have any specific question feel free to PM me or just ask in here...