From the ASB website:
Special offer for Vodafone prepay customers
The first time you use HotLink to top-up your prepay mobile, Vodafone will double your first top-up to a maximum value of $50. Offer applies from 10 July to 5 August 2007. The double up will be applied to your Vodafone prepay within 5 working days of your first HotLink top-up.