Kmart Optus Nokia Lumia 530 (Locked to Optus) $29
Saw this at my local Kmart store, pretty cheap imo. EDIT sorry, this was in Sunnybank Hills Kmart, 5+ stock when I was there.
Locked to Optus, if you activated the sim for less than 6 months, $80 unlocking fee applies. After 6 months its $25, however ebay unlocking options are available too.
As other members have suggested(thank you), this makes a great $29 GPS in your car. Simply download the offline Australian map into Here Drive(Nokia's Navigation software) https://www.youtube.com/watch?v=D1k0r5YD9PQ
Specifications - WINDOWS 8.1 MOBILE
Qualcomm Snapdragon 200 (Quad Core 1.2Ghz)
4GB Internal with microSD card up to 128GB
5MP Rear Camera
Windows 8.1 OS
microSIM Card size
UPDATE: I was able to unlock my Lumia 530, I had a active $2 Optus SIM number(active for over 6 months) lying around so I emailed them and they gave me an unlocking code within 2 days. If you have any friends/family member that has a prepaid Optus SIM active for over 6 months, use their number and email them, I'm sure it'll work too.