I've used this twice on webjet directly http://www.webjet.com.au/. I don't believe you actually need to register your card.
Save $75* on your hotel stay with Webjet when you spend AUD $500 or more in one transaction. Choose from over 1 million hotel rooms on sale every day, in destinations across Australia and all over the world. Valid at participating hotels only. See terms and conditions for excluded properties. Simply book & pay for your hotel before 30th April 2015 and travel before 29th February 2016.