As of 30th September 2014, the NZ Domain Name Commission has made second level domains - like the '.co' in '.co.nz' and the '.org' in '.org.nz' – optional so you can now register names with them, without them, or both.
For example, www.<yourdomainname>.co.nz could now be www.<yourdomainname>.nz
The NZ Domain Name Commission has put together a comprehensive website on this change which can be viewed here.
Depending on when you first registered your domain name, you may get Preferential Registration or Reservation (PRR) to claim the shorter version of your domain name, or it may be available on a first-come, first-served basis.
It's therefore important for you to get onto registering or reserving your shorter '.nz' domain name if you want to claim it. The final day for Preferential Registration or Reservation (PRR) to claim the '.nz' version of your existing domain name is 1pm, 30 March 2015. All key dates can be found here.