It's not just about the kids sucking. They need to be milked out dry twice daily to keep production up and it also depends on the available nutrients, so they get all the minerals they need plus protein and carbs will help them live up to their potential. Then there is the genetic factor. Some goats just dry up soon after kidding and some of the better bred
dairy goats will go 10 months or sometimes years without the need for rebreeding so they "freshen" again. There are many herbs that increase lactation, and some that dry up an animal. Also good protein
hay in addition to regular hay as it is sometimes too rich alone, such as alfalfa, really good pastures with lots of minerals on the soil, things like chicory or mulberry leaves and probably a hundred other things, will help.
Weather conditions play variable roles too. If you are in North Island, the weather, of
course will be milder than South Island and you will have more year round pasture, and if you can keep those rotated so they never graze them down too much, that is a bonus. Fresh forage is always better than just hay and grain. In NZ you
should have great soils anyway, so may not need to do much in the way of mineralization because they are so mineralized already from volcanic dust. Good luck in your endeavors!
Melba