The reason the British did not just take over was that a strong humanitarian movement centred on the churches influenced British colonial policy and this meant the Colonial Secretary Lord Normanby genuinely tried to treat New Zealand inhabitants better than happened in earlier colonisations.

Hobson's brief was "to treat with the aborigines of New Zealand in the recognition of Her Majesty's sovereign authority over the whole or any part of those islands which they may be willing to place under Her Majesty's dominion" but only with "the free intelligent consent of the natives"

Lord Normanby's brief to Hobson here > http://nzetc.victoria.ac.nz/tm/scholarly/tei-Mac01Comp-t1-g1-t5-g1-t2-g1-t6.html