There are a few mountains in the central North Island of New Zealand that start with "T", but the most famous and likely the one you're thinking of is Mount Tongariro.

Here are the other possibilities:

* Mount Taranaki: This is a prominent volcano on the west coast of the North Island, not the central region.

* Mount Tapuae-o-Uenuku: This mountain is in the Tasman District, on the South Island, not the North Island.

Let me know if you're looking for information about another mountain!