If you're on a point-to-point ticket ie NZ to Japan, without touching North America, then you're restricted to the 20kg limit if you're travelling in Y. However, you may try asking your travel agent (if you're using one) to intervene by asking for a special weight increase allowance through their sales representative. High volume agents with respective airlines have clout with their sales reps.

It may not be that much more ... but at least you'll be given some extra allowance.