Kiwisaver.

I haven't joined yet, although I do intend to.

Reason:
1. I only work part-time, and I need all of the money I generate for personal expenses.
2. I also have a small amount of personal debt to pay off first.

However, I plan to join in November, when I am again working full-time over the University Break.

However, it would also be good if one could divert funds from Kiwisaver in the same manner as mortgage diversion for student loan repayments. While, economically, it might be a sucker punch to pay off interest-free debt, there remains a great deal of skepticism on my part that this policy will survive a change in government.