There are a few methods:

You can make a dummy booking on Air NZ and note the seats available (in each class) - but this depends on status as not all seats are available are visible. In any case some who have booked may not have selected a seat.

Some 3rd party travel websites offer a 'view the cabin' option which queries the seat map.

A subscription to a tool such as KVS or ExpertFlyer allows querying of loads (for numbers) and seat maps.