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.
"That's pretty solid demand. We're pretty pleased with that," he said.
The company is unable to give any steer on when it will offer 4G services outside Auckland, Wellington and Christchurch as it is still waiting for the auction of 700MHz spectrum to be completed.
This spectrum is being freed up from the switchover to digital television and has been auctioned off to New Zealand's three mobile companies.
Telecom has lagged behind rival Vodafone in getting a 4G network live. Vodafone launched its 4G services in parts of Auckland in February and according to its website now also offers it in parts of Palmerston North, Wellington, Taupo, Wanaka, Queenstown, and Christchurch.
2degrees - the smallest and newest mobile company in New Zealand - has yet to announce a 4G launch date.
2degrees' chief executive Stewart Sherriff told the Herald last month that 4G was "in the planning mix" and indicated the services could be launched next year.