Only Telecom post-paid customers can get a Telecom SIM card, and then it can only be used for international roaming (currently) to GSM locations within Telecom's roaming partner alliance. If you have a Telecom NZ phone, you can take it to your local Telecom Store or Dealer to be connected (no SIM required).
From November 2008, all going to plan you should be able to purchase a Telecom SIM card for local use. You will need a handset that uses the 850 MHz GSM band, and the 2100 WCDMA band through.
I wonder when Telecom's WCDMA network get going we are going to see the situation here as in Ozz where 90% of the fones you buy from the carrier are locked and cost about $70 to unlock in the first 6 months??