If you still haven't had this sorted, make sure you visit our Vodafone Mates for all the info you need! Specifically the questions at the bottom of the page:
How do I get a 500MB nonus for being under 25?
"Easy as. Once you’ve designed your plan and made your first top-up, you can confirm your age through Facebook. If you didn’t do it when you signed up, just log into My Vodafone at any time to get it done. When you view your plan, you’ll be given an option to confirm your age through Facebook.
If you don’t have Facebook, you can confirm your age by showing a valid Photo ID accepted in New Zealand (Passport, Student ID, Driver’s licence) which shows your photo, your full name and date of birth at any Vodafone store.
Once you’ve done either of these, you’ll get your first 500MB bonus immediately without needing to top up $20 or more."