Sorry, wrong words. You get a "write-off" under the circumstances you said. However you get an interest "REDUCTION" if you earn more than $15,132 and:
"your base interest is more than 50% of your repayment obligation for this income year, and
you are a New Zealand resident for tax purposes."
Bascically this means that for every repayment you make, at least HALF goes off the principal, the other half goes off the interest that got charged for that period, and any interest over that is wiped. Labour brought that policy in about a year and a half ago.