The one and only advantage of AirNZ's Airpoints programme is that you can use it to buy any available seat at a rate of 1 airpoints dollar to NZ$1. Whether or not that's a decent deal really depends on the prevailing price on the day, but it does allow you to avoid searching for award availability. On shorthaul economy class, that can sometimes be a reasonable deal.
So, if you're really keen to use less cash, you *might* find it useful to convert some of your MR points (perhaps via an intermediary like SPG) into Air New Zealand airpoints. You'd want to check the transfer rates pretty carefully first. Also, a double transfer like that could take a while so prices might go up in the meantime.