Heya...
I am an aussie who married an american citizen and went through the process mentioned by the previous poster- the fiance visa...so I will just give you my thoughts on the subject..i'm not a lawyer though this is just my personal opinion..ok?
As you mentioned there is no guarantee of having your B2 extended and to be honest, if the consulate knows that you are planning to get married while in the USA they may be reluctant to issue a B2. They usually want you to prove that you ties back to australia and that you will return.
If you were to go down the road of the K1 (fiance visa) you MUST be married within 90 days of you arriving in the States which is not what you are after as you mentioned you wanted to spend a year there first.
You can get married in the States while visiting but I wouldn't advise it because when it comes to adjusting your status to a resident you may have to prove that you did not intend to get married, that it was a last minute decision. I think if they think it had been your intent all along to get married it is in violation of the conditions set on a visitors visa.
OK! So you have two choices really..
K1 (fiance visa)- it's a relatively easy process, you don't need a lawyer to complete it, my fiance and I did it on our own and I am now living in the States waiting to become a permanent resident. You can START this process while you are still in the States as well, but you will have to go home to Australia to go to an interview at the consulate in Sydney or Perth (whichever is closest) to obtain the visa. It should only take about 3 months or so to get it all done. If you would like some more detailed info about this one then head over to
http://www.txdirect.net/users/mike38/k1faq.htm
which is a wealth of information.
I-130 (petition for alien relative)- this one is if you have already married your fiance in Australia and she goes back home and asks that you be brought over to the USA as her hsuband. I did not go through this process so I know only small bits about it. I know that it can take alot longer than the K1 visa. I don't have any URLs for this one- sorry.
And finally,
http://www.ins.usdoj.gov/graphics/index.htm . This is a pretty good website and may be able to answer some of your questions as well.
Hope that was a bit of help..
Christie