Or we could just quote their website at them - doesn't say anything about sending it in before hand:

How to get it

1. Fill in a form at Whakapapa or Turoa Customer Services.  Or even better.. SAVE TIME... by printing out the pdf form below and filling it in before you get here. 
2. Both of you need to come to Customer Services at Whakapapa or Turoa to redeem the discount. The Season/5 Season/Life Pass holder must have their pass and their friend will need photo ID.

Bring a friend form Adobe Acrobat Document: 36.7 KB, 5 seconds @ 56kbps