Brits went with 999 because it was easy to convert payphones from allowing 0s to be dialed for free (allowing users to call the operator) to allowing 0s and 9s to be dialed for free. In the UK, the system was that X pulses would be sent to send the digit X (10 for 0). NZ was using British exchange equipment that would recognize three sets of nine pulses as the emergency number, but NZ dial phones used the reverse convention (10-X pulses) so that mapped to 111.
Comments
Brits went with 999 because it was easy to convert payphones from allowing 0s to be dialed for free (allowing users to call the operator) to allowing 0s and 9s to be dialed for free. In the UK, the system was that X pulses would be sent to send the digit X (10 for 0). NZ was using British exchange equipment that would recognize three sets of nine pulses as the emergency number, but NZ dial phones used the reverse convention (10-X pulses) so that mapped to 111.