Upgrade costs and practicality
Aircraft operating in New Zealand airspace currently have a diverse range of navigational capabilities. This diversity, coupled with a wide mix of aviation activities, a high level of non-commercial operations and an older aircraft fleet, mean that not all participants will have the level of equipage to meet future requirements and that upgrading costs will vary greatly.
A balance will need to be struck between the needs of the operators who are driving towards an exclusive PBN environment (which will allow best cost savings) and the needs of some participants in the aviation system, in particular some in the general aviation community, for whom cost may outweigh benefits until equipment costs reduce.
In addition, there are some practical implications to take into account, New Zealand only has a small number of avionics engineers experienced in the new technology and is reliant on supply of equipment from overseas. Time will be needed to allow operators to arrange for upgrades and there may be impacts on the operation of both existing and new aircraft systems.
Just read this in the Southern Skies project by NZ CAA. It appears like a more balanced approach.