Coles / Bi-Lo have 20% off Amaysim recharges betweeen Wed 11th and Tues 17th Jan. This makes the Unlimited + 4GB Data plan under $32 per month!
It's in the local Sydney leaflet for both Sydney and Melbourne, so I assume it's nationwide.
Limit of 3 recharges per transaction, so you can stock up! Note: I have not checked how long the vouchers actually last for, so don't go too berserk!
EDIT - liuzhen has posted below that the vouchers expire 1st November. And I just realised that I THINK it may be possible to simply load the credit into Amaysim and just leave it there (But I cannot find expiry dates on their website).
ADDED INFO - The standard retail recharges are $10, $15, $20, $30 and $50. But the lady at Coles just gave my girlfriend a $39.90 Unlimited voucher for $31.92 so she bought that instead.
This is the same price as http://www.ozbargain.com.au/node/61330, but available to existing clustomers instead of just new sign-ups.
It should say on the voucher when it expires, usually you do have quite a long time to activate it. Purchase accordingly :)