"A sweet $10 bonus will be yours if you’re one of the first 10,000 Cash by Optus customers. Not only that, but we’ll add another $5 after your first payWave payment. All you need to do is register before 31 December 2015"
Cash by Optus is available to Optus customers who are on a monthly mobile plan and who have a compatible Android mobile phone.
Appears to be limited to optus contract customers - from the FAQ:
Can I get Cash by Optus™ if I’m a prepaid Optus customer?
A. No, Cash by Optus™ is currently only available to Optus customers who have a monthly mobile plan and who receive a monthly mobile plan bill.