A low-lying paddock in Mosston Rd will become a wetland to hold run-off from manufacturing businesses in Whanganui's Westbourne Industrial Area.

But it will not reduce the high groundwater that vexes residents in parts of Castlecliff.

Whanganui district councillors agreed at a meeting on May 26 to buy the 2ha paddock at the corner of Mosston and Mill roads. The cost of $230,435 was negotiated in a fair-market settlement under the Public Works Act.

The owner was ready to sell, Whanganui District Council senior stormwater engineer Kritzo Venter said, and it made sense to acquire the land now.

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Concept plans for it are being worked on. More land may yet be required, and the total cost of slowing the industrial run-off is estimated at $780,000, funded by developer contributions.

The Westbourne Industrial Area is 120ha of undeveloped land reached from Mosston Rd and zoned manufacturing. There has been keen demand for property in Mill and Rakau roads this year.

Venter said 90 to 95 per cent of the area may eventually be covered in hard surfaces such as roofs and paving. In storm and flood conditions a lot of water will run off and the council has invested in stormwater infrastructure to channel it.

The paddock at 85 Mosston Rd will always be damp, Venter said. Normal rainfall will drain away slowly toward the Titoki Wetland.

Extreme events like the June 2015 rainfall that caused the Whanganui River's biggest flood would add hundreds of cubic metres of water. It will be accommodated inside an earthen stopbank, and drain out over a spillway into a Mosston Rd pipeline before being pumped to Wharf St and the river outlet.

The paddock is perfect for the wetland, Venter said. It is low-lying and little earthwork will be needed. It will suit the wetland plants needed to treat the industrial run-off.

It will attract wildlife such as pukeko and may eventually have a cycleway alongside it. Most of the time it will be damp, but not have standing water.

"It will actually add quite a bit of value to that area. Especially iwi are very supportive of this, restoring the mauri, the life force of the water as it comes through," Venter said.

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The paddock used to be very dry, deputy mayor Jenny Duncan said. The groundwater level has come up considerably.

Cr Charlie Anderson asked whether the wetland would reduce the high groundwater level that annoys some Castlecliff residents.

It will not affect that and will only deal with run-off from the industrial area, Venter said.

"You can expect some feedback from the locals then," Anderson said.