Natural wetland habitats are becoming rare in Auckland, and constructed wetlands aren't filling the gap.

Construction has just got underway on a new wetland at Lowtherhurst Reserve to collect stormwater from the city's northwestern motorway.

Well-constructed stormwater wetlands are supposed to filter water run-off and provide a habitat for wildlife.

But Forest and Bird Auckland manager Nick Beveridge said many of the city's constructed wetlands are too toxic for birds.

"The main threats are drainage or agriculture and urban development encroaching on wetland areas."

Te Henga Wetland, one of the city's few remaining natural wetlands, is an important habitat for birds like the Australasian bittern.

About 90-percent of New Zealand wetlands have been destroyed -- in Auckland that number is 97-percent.

Beveridge said constructed or restored wetlands can provide a replacement habitat but they often don't.

"If they replicated the conditions that a natural wetlands provide, then they could but often that's not the case."