Today Telecom NZ officially announced that its 4G mobile network will go live overnight tonight in parts of Auckland, Wellington and Christchurch.

4G will be available at no additional monthly cost to Telecom's prepay and on-account customers. Vodafone NZ's 4G service is only available to its on-account customers, and costs an additional $10/month. Mobile carrier 2degrees does not offer 4G at this time.

To use 4G, customers must have a 4G capable smartphone or other device, and a 4G SIM card. This means that existing Telecom customers will need to upgrade their SIM at a Telecom store before they are able to use the new service.

Telecom's 4G network is entirely LTE Category 4, which offers theoretical speeds of up to 150Mbit/s download and 51MBit/s upload with Category 4 devices. Most 4G/LTE smartphones currently on sale in New Zealand are Category 3, which offers theoretical speeds of up to 102Mbit/s down, 51MBit/s up.

Competitor Vodafone launched its 4G network to support Category 3 devices, but announced Category 4 support in some areas on 10 October this year.

Telecom coupled its 4G announcement with promotion of it existing 'free national WiFi' offering, a Wi-Fi network that currently includes 750 hotspots. On-account Telecom mobile customers, and Telecom prepay customers on $19/$29 prepay packs, will have access to 1GB per day of free traffic on that network.

Phones configured to use the Wi-Fi network will do so instead of 4G when Wi-Fi is available, potentially lowering the paid-for data usage of 4G customers. The greater the extent of Telecom's Wi-Fi network, the greater the effect of this. Currently installations are tied to Telecom phone booths, and in public locations such as transit hubs.

Speeds attainable over Telecom's Wi-Fi network will depend on distance from the hotspot, number of concurrent users, and interference from other wireless networks in the area. It is unclear how the performance of the Wi-Fi network will compare to Telecom's 4G offering.