Noticed this at the bottom of the new Qantas Cash site - http://www.qantascash.com - yet another way to get a free QFF membership.
If you're Australian resident 16 years of age or older you can receive complimentary Qantas Frequent Flyer membership, saving you $82.50. Simply complete the following online application form and you will receive your Qantas Frequent Flyer membership number instantly, so you can start earning Qantas Points straight away.
Your new Qantas Card will be sent to you in the post (please allow 14 days for delivery). Once it arrives you can apply to activate the Qantas CashTM facility to start earning Qantas Points on eligible transaction.
There always seems to be a way to get free membership, some of the codes in this post probably still work as well.