I am going to assume these work the same as other Qantas vouchers
Yes you will get the full fare back, but it's going to be in the form of a booking sitting on your account from QZW Airport - QZY Airport
The expiry date will still be the date of the flight booked, so if you want to do this, I'd be booking the flights as far possible in advance, eg buy vouchers now, they expire on 13th December 2013, you wait until about this time to book something.
So you then book a flight for late November 2014 and then you have any time from November 2013-November 2014 to use them
Already purchased $5000 here, and will now wait a week or so and purchase another 5k for a lazy 120,000 points (100,000 from QF plus 20,000 from Amex) then another xx,xx_ points for whatever I use the vouchers for
Thanks Qantas