Simplex means talking directly from one radio to another on a single frequency, with no repeater in between. It's how you work nearby stations, test equipment and keep talking when a repeater is busy or off air. The frequencies below come from the WIA band plan.
1The calling channels
146.500 MHz: 2m national FM simplex call channel.
144.100 MHz: 2m SSB and CW calling (New Zealand uses 144.200).
439.000 MHz: 70cm national FM call channel.
439.200 MHz: digital voice call channel.
432.100 MHz: 70cm SSB calling.
2Where to move after contact
Analogue simplex segments on 2m are 145.2125–145.4875, 145.6625–145.800, 146.4125–146.6125 and 147.3875–147.5625 MHz. Digital simplex on 2m is 144.700–144.8875 and 145.0625–145.2125 MHz. On 70cm, analogue simplex is 438.9375–439.0375 and 439.2375–439.6125 MHz, and digital simplex is 439.0375–439.2375 MHz.
It's common practice to keep calling-channel contacts short: make contact, then agree on another simplex frequency and move. This is a convention rather than a written band plan rule.
3Frequencies to avoid for simplex
Satellite: 144.000–144.025, 145.800–146.000 and 435.000–438.000 MHz.
Beacons: 144.400–144.600 and 432.400–432.600 MHz.
APRS: 145.175 and 439.100 MHz.
Repeater inputs: transmitting there can key up a repeater you didn't mean to.
4Make a simplex call
Listen on the channel for a while. If it's clear, call with your call sign, for example asking whether anyone is listening on the channel. Identify at the start and end of each transmission and at least every 10 minutes (class licence, clause 12).