Employees have the option of choosing a KiwiSaver scheme of their choice at any time. If you don't choose a scheme yourself, you can either become a member of the scheme preferred by your employer (if one has been selected) or the IRD can randomly assign you to one of the six default schemes.
Existing KiwiSaver scheme members also have the option of moving to a different KiwiSaver scheme at any time, but you can only be a member of one KiwiSaver scheme so any accrued benefits would need to be transferred to your new scheme.
It's important to think about choosing a scheme yourself, for several reasons:
There are really two main aspects to choosing a KiwiSaver scheme: (1) the provider of the scheme, and (2) the characteristics and range of the investment funds offered by the scheme. For help, see Choosing an Investment Fund.