You may be in (some) luck!
Both Qantas and Virgin Australia stipulate that you can only claim miles for flights taken up to 30 days before becoming a member. And retrospective mileage claims are not possible with Jetstar.
However... according to the Southwest Airlines website:
When I enroll in the All-New Rapid Rewards program, can I get points for flights I completed prior to enrolling? Yes, you can request past flight points for flights completed in the past 12 months by logging into your Rapid Rewards account and choosing Request Past Flight Points from the Quick Links drop-down box at the top of the page.
So, you may be able to claim Southwest miles. As far as I know, they can only be used on Southwest, so will only be useful if you plan to return. But hey, if they're free...
I have not been able to find much information about American Airlines, other than that you can claim American AAdvantage miles for AA or partner airline flights taken in the past 12 months. I don't see any clause about needing to have been a member when taking the flights. So, if I were you I would sign up to AAdvantage (it's free anyway) and fill out the online form. You should be able to (possibly) claim any American Airlines or Qantas Flights. (Also any flights on other oneworld airlines like LAN, TAM, British Airways etc.) No harm in trying. If you are able to get some miles, AAdvantage miles are actually quite useful for travel within Australia/NZ on Qantas so certainly worth the effort.
Ordinarily I would suggest crediting Hawaiian and Delta to Virgin Australia but that won't be possible in this case. Hawaiian and Delta are partners though so you should be able to credit flights from both airlines into one airline's program. Hawaiian requires the flight/s to have been in the past 120 days though. And Delta have the 30 days for new members clause. You may be able to credit the Delta miles to another SkyTeam program but depending on the number of miles you actually flew, it may not be worth the effort.
)
