It appears that a number of aircraft had been chartered by a single organisation to ferry people to Wellington [and other places during the preceding week].
Some of the pictures I got follow. Top to bottom are:
Piper PA-31-350 Navajo Chieftain, ZK-VIP, registered to Air West Coast of Greymouth
Cessna 402C, ZK-VAD, registered to Ridge Air of Blenheim
Cessna 404 Titan, ZK-NDY, registered to Ashburton Air Services
Canadair CL600 Challenger, N604AU, registered to ANZA Aviation NA LLC of a USA address
General Dynamics Convair 440/580, ZK-CIF, registered to Air Chathams