West Coast landowners have wiped out more wetlands in the past 20 years than landowners in any other region, Forest and Bird claims.
In a statement released to mark World Wetlands Day, conservation groups said aerial images from around the country supplied by Landcare Research, showed wetlands on private land were still disappearing at an alarming rate.
"The images we've released are a few of many examples where wetlands are being erased right under our noses," Forest and Bird freshwater advocate Annabeth Cohen said.
"With more than 90 percent of New Zealand's original wetlands already destroyed, what's left is incredibly important."
The West Coast is the largest wetland region in New Zealand, with nearly 84,000 hectares of freshwater wetlands, 14 percent of them in private ownership, Forest and Bird said.
But since 2001 Coasters have damaged the largest area of wetlands, with more than 10,700ha partially destroyed in that time.
The National Wetlands Trust said more recently (2012-18) Westland lost 153ha of valuable freshwater herbaceous wetland, coming second only to Southland, where 755ha disappeared.
Most of what has been lost has gone into grassland for dairy farming, according to Trust chief executive Karen Denyer.
"Wetland destruction happens insidiously over time, starting with a few access roads, then some drainage ditches; a spot of vegetation clearance and little by little the wetlands dried up," Denyer said.
"These were areas of plants like carex, raupo, and flax and big sphagnum bogs."
New Zealand could not afford to lose more wetlands, she said, especially at a time of climate change.
Healthy wetlands protect people and wildlife, the conservation groups said.
"They act as coastal buffers, shielding our lowlands from storm surges; they reduce floods and relieve droughts and they absorb and store carbon.
"We assume that a proportion have been legally destroyed throughout New Zealand due to poor rules at the regional level - this is because national regulations protecting wetlands have been non-existent," Annabeth Cohen said.
West Coast Regional Council chief executive Mike Meehan said there were more than 200 protected wetlands in the region's Land and Water Plan, covering 7000ha on private land, and 60,000ha on Crown land.
These had been identified during an extensive RMA plan change spanning almost a decade of appeals and mediation through the Environment Court.
"If the [Forest and Bird] figures are correct - that 153ha of wetland were "developed" and "lost" in the period 2012 to 2018, these areas would have been subject to the RMA framework that allows for a balanced approach to assessing development in these areas," Meehan said.
"We are confident in our planning framework to protect those areas which are significant, either in vegetation or as habitat of indigenous fauna."
There were also pitfalls in using aerial photography to define wetlands, he warned.
"About 270 individual West Coast landowners were initially affected (by the wetland identification process) but these areas were mapped using aerial photography and accepted by the Environment Court - something the regional council contended would lead to inaccuracies."
'Ground-truthing' (inspecting the land at ground level) over several years had proved the council correct, and a number of areas wrongly identified had been removed from the plan.
The council had worked hard to develop its rules and policies for wetlands through plan changes, mediation and the Environment Court, and they reflected the social, economic, environmental and cultural outcomes sought by local communities, Meehan said.
"We understand the importance of wetlands in the national context, but if the West Coast wetlands are that important in the national context, the Crown needs to purchase these in the national interest."
Local Democracy Reporting is a public interest news service supported by RNZ, the Newspaper Publishers' Association and NZ On Air.