The BigPipe website has what I assume is an error for the refer a friend promo. If you visit the FAQ https://www.bigpipe.co.nz/#/home?a=faqs , check out "Terms", and look at the entry for "Where can I find the terms and conditions for the iTunes Refer-a-Friend promotion?" it says "between 1st September 2014 and 1st December 2014" or similar in a few different places which suggests the promotion has ended.
However per a February Twitter post I just noticed, it seems this promotion is still running. https://twitter.com/BigPipeNZ/status/563864688400674816 .
May I suggest you update the T&C as plenty of people like me may assume it's over and not bother trying/asking? (And yes I know this isn't an official support forum, but for this sort of thing, it seemed more likely to reach the right people faster.)
In my case it's too late :-( Both me and my friend were on BigPipe together for over a month, but now it's only my friend. I moved to Spark as I wanted a phoneline and Spark door to door offered me an unlimited VDSL+phoneline deal that was better than even BigPipe + VoIP line. Still I'm sure others will benefit, including those who may not have referred a friend otherwise.
(I guess this is similar to the first month free promo you've extended a few times. The T&C for the first month free also needs an update, but that's a bit less confusing since it's advertised when signing up etc.)
Cheers