There are a large number of bird species that use this area for nesting, brooding, feeding or resting. They are all detailed in this report (pdf 937 Kb) commissioned by the Department of Conservation. Reaction to this report made the front page of the Herald ('Plan for new town spells disaster for fairy tern, says DoC') and was on Radio New Zealand's Checkpoint programme ('Subdivision proposed near breeding site of NZ's rarest bird')
The two main species these experts are gravely concerned about are the New Zealand fairy tern and the northern New Zealand dotterel.
Forest and Bird have also issued a press release raising their concerns ('Coastal development threatens rare birds')
The New Zealand fairy tern has only 11 breeding pairs left in the world - and 5 of them nest at the Mangawhai Sandspit (immediately North of the propsed development) and use the area around Te Arai stream (in the middle of the development) as a post-breeding flock site. It is New Zealand's rarest breeding bird.
The northern New Zealand dotteral is "Acutely Endangered" with about 700 pairs left. 30-40 pairs nest on the Mangawhai Sandspit and a futher 8-9 at Te Arai stream.
The developers have come up with an Action Plan to "reduce the impact" of the development. DoC's report (pdf 937 Kb) basically says that the plan is inadequate for the massive influx of people and predators that would occur if this development went ahead.
It sums up thus (emphasis is mine):
"There must be a very real concern that the elements of the proposed Action Plan that relate to human activities and disturbance would simply be overwhelmed by the number of people likely to be on the area's beaches. In that case, there are bound to be significant impacts on fairy terns and New Zealand dotterals; in the case of fairy terns, these are potentially catastrophic for the taxon. Should such impacts occur, it seems unlikely that the situation could be retreived."