Do not come on the visa waiver, and marry, and try to adjust as a one-step. With your past, if it comes out that the trip was with the intention to immigrate, you could be charged with visa fraud, and end up needing a complicated waiver-- or being denied-- and having a complicated case. Some lawyers may advise you that you can do this-- and it is true that some people do enter on a B1/B2 or visa waiver, marry and adjust-- but why risk it? You do not know who you will get for your interview-- and especially with the prior visit, the immigrant intent issue would put you in a corner.

Do the K1 fiance visa. You file it with your local service center. In 1-3 months, you receive an approval notice if all is clear. Then, your fiance will receive Consulate Paperwork in Australia to complete and return to the U.S. Embassy. After that, an interview is set up. If all is well, your fiance enters the US, and you marry within 90 days, not 91+ but 90 days. Then, you file your I-485 adjustment of status packet with all the supporting paperwork. Your spouse receives Conditional Permanent Residency for 2 years, and then 90 days from the date of the actual marriage interview-- a form I-751 is filed to lift the conditions off the CPR.
This is of course, if all the proper forms are completed, and documentation in support is there. If people are educated, and either never married before, or have the death/divorce documents from any prior marriages, the processing may go more smoothly.

Yes, it could take 6 months-- but it shouldn't take as long as a year if there is no delay on your parts. You are applying from First World countries here and this can make a huge difference. For one thing, the paperwork can all be obtained and authenticated and no need for translations-- factors such as that.
K-3 is to bring a spouse in and this is done as it was quicker to bring a fiance than a spouse in the past, so that is part of the background on K-3. You can visit each other--and the immigration process can really test a relationship-- but you need to gather your calls, emails, photos and all so work on that.

Note that my view is very conservative and with the view to shoot straight and close the file. The first bite at the apple strategy, or best offense as a good defense.

The previous poster supplied important information for consideration. The following is another approach that may be appropriate for you and your fiance to consider.

After a careful analysis of details relating to your fiance, including the strength of the evidence supporting the proposition that he entered the U.S. without "immigrant intent," then it may be possible for you and he to become married within the last few weeks of his authorized stay, and then immediately apply to adjust status to become a Lawful Permanent Resident (to get a "Green Card"). Simultaneously he could apply for Advanced Parole (a "travel document") that would allow him to return to Austrailia and re-enter the U.S. while the adjustment of status case remains pending.

As you may have gathered, before taking action it would be important to obtain immigration legal advice from an attorney who carefully has reviewed the specific details of your fiance's situation.

[Note: Consistent with Avvo policy, this communication is intended as general information and not specific legal advice, and this communication does not create an attorney-client relationship.]

David N. Soloway
Frazier, Soloway & Poorak, PC
1800 Century Place, Suite 100
Atlanta, Georgia 30345 www.fspklaw.com
404/320-7000 877/232-5352 dsoloway@fspklaw.com

What I would add to the poster following my approach with a different approach is this:
The "question" states that the finance is returning home next week. So, unless the marriage and application would be filed in a few days, and the Advance Parole picked up at the Embassy in Australia (if granted, and allowed to be picked up there)-- it seems difficult to work with the time lines here and the first visa waiver.

I would be quite hesitant to come back on a second visa waiver and then argue that was without "immigrant intent," as the first trip is when the parties met-- and they will be under oath discussing the intent to marry, and when the engagement was, and ring, and families meeting and emails and phone calls. When they decided to get married, and since they did decide all this during visit #1, then visit#2 has immigration intent, and the parties and under oath-- and might get caught in a bit of inconsistent facts-- and then, if a waiver would be needed, this would begin to be costly, and time-consuming.

Therefore, even if it could be done, and slip through, I respectfully disagree, and would do it with a K-1, and legal non-immigrant hybrid visa. Yes, some immigrants choose to ignore such a strategy, and most do get processed, however, is it worth the risk?