BirdingNZ.net

The Whitehead is found only in the lower North Island and from the same species as the Brown Creeper- Pipipi and the Yellowhead- Mohua, which are both only found in the South Island.

Taken at the DOC camp, Kuripapango on the Napier-Taihape Road- there are good flocks of whitehead in the surrounding bush and along the Ngaruroro River margins.

Whitehead- Popokotea by flyingkiwigirl, on Flickr

They are relatively common throughout that area, we always see big big flocks in the Kaingaroa forest (Napier-Taupo highway) lovely little birds, they along with Brown-Creepers have to be some of my favourite endemics! Awesome photos as always Shellie.

I was on Rangitoto Island a couple of weekends ago, and nice to see a couple of whitehead flocks in the bush up near the summit, including several fledglings. They seem to be settling in well. Of the other birds, saddlebacks are still fairly thin on the ground - had good views of one and heard a few others. Also saw a few red-crowned parakeets, but recovery of the island's birds is obviously still in the early stages.

I don't quite follow your first sentence. I would say they are most common/widespread in the Central NI, and are common in places north of Auckland where there are few predators. Certainly not restricted to the lower NI.
And "from the same species" as brown creeper and yellowhead??

Hmm...this will teach me to spend a little more time in research (or not type an intro - I usually do research, but was on limited internet connection when posting these- driving 13kms to find reception & using my phone hotspot limits the amount of time I have to check facts. AND travelling full time in our RV limits the amount of bird books I have on board- weight being an issue. I have two small books- one says 'they are of the same genus - 'Mohoua'. I have worded it incorrect.

And I got the distribution information from NZ Birds Online- again speed reading & condensing incorrectly it would seem

Distribution and habitat

The whitehead occurs only in the North Island of New Zealand. Its mainland distribution is south of a line connecting the Pirongia Forest, Hamilton and Te Aroha. It is locally common though patchily distributed south of this line, and to the north is limited to offshore islands and one fenced translocation site. Whiteheads are naturally common on Little Barrier and Kapiti Islands, and have been successfully introduced to Tiritiri Matangi Island, Motuora Island and Tawharanui Regional Park in the Auckland area, plus Maungatautari, Mana Island and Karori Sanctuary. Attempts to establish whiteheads in the Waitakere ranges (Ark in the Park) and at Cape Sanctuary (Hawke’s Bay) are continuing, and in 2011 or 2012 they were released on Moturoa Island (Bay of Islands) and Motuihe, Rangitoto and Motutapu Islands in the Hauraki Gulf.