Network offers mobile internet 10 times faster than before.
Telecom switched on its 4G mobile internet services overnight in Auckland, Wellington and Christchurch but still can't say when customers outside those centres will get access to the faster services.
Telecom customers - both on pre-paid or on-account plans - can now upgrade to 4G services for free but will need a new SIM card and the right kind of smartphone, tablet or portable modem.
4G services offer internet speeds on these devices that are around 10 times faster than on a third generation network.
Telecom retail chief executive Chris Quin said the company sells 18 devices - such as the iPhone 5 or Samsung Galaxy S4 - which are 4G capable and that 40,000 customers had pre-ordered 4G SIM cards.