Corn that is specifically flour corn, needs a long season. Most parts of NZ can grow flour corn. It needs to be left in it’s husk until the husk is totally brown and dry, but it does not necessarily need to be left on the plant until it is totally dry. Like most other seeds, they reach a time when the seeds are fully developed in size, and they begin drying. When they begin to dry the pods (husks in this case) usually change colour, usually going a duller colour or actually losing colour . At the point the seed pods or husks are totally dry the seeds will be fully mature.
Once you can see that the husks are changing colour towards dry and brown, then you can harvest the cobs and put them in a dry warm airy place to finish maturing until the husks are totally dry. Be very careful about rats and mice!!
Once the husks are dry you can remove the cobs from the husks at which point they will be fully mature, hard kernels and holding their full colour.
You can now choose to store the kernels on the cobs, which is how corn was traditionally stored, usually in something like a corn crib. Keeping the kernels on the cob is the easiest way to keep the corn weevils from getting into the kernels. The kernels can also be removed from the cobs, which then means they take up far less space, but they need to be mixed with wood ash, or diatomaceous earth to stop the weevils destroying the kernels. In this case I would store them in either a fired earthen crock with a lid, or glass or a container that keeps rats and mice out.
From there the corn kernels need nixtamalizing and that is well covered in our online Traditional Food Storage Workshop and also Change of Heart our cookbook, along with many options for preparing the dry corn as food