A wetland at the bottom of a dairy farm in South Wairarapa is reducing most of the nitrates being drained from the paddocks. The project at Kaiwaiwai Dairies is nearly a decade old and one of the owners takes us on a tour.

Drinking from the drain flowing out of a corner of his South Wairarapa dairy farm is not such a crazy idea, Aidan Bichan says.

If it weren't for the wildlife living in Kaiwaiwai Dairies' three-quarter hectare wetland, its co-owner has only a few qualms about taking a sip.

He took Country Life on a tour of the wetland, established nearly a decade ago.

Take a tour of Kaiwaiwai Dairies' wetland with Country Life

It was designed with advice from Greater Wellington and NIWA after several iterations and dug by an earth-moving contractor who's a fellow shareholder in the farm.

"He understood the issues around dirt."

Six metre-wide channels were dug in the carbon-rich peat and the soil pushed up to form banks in between 12 tightly squashed up curves.

The banks are now heaving with harakeke and native trees and shrubs.

Water-filtering plants were added and over the years they found raupo did the best job of "polishing up the water."

The aim was to remove nitrates from the water drained from 200 hectares of pasture.

In earlier times the pollutants from cow urine, fertiliser and plants would have ended up in the Wairarapa Moana.

While nitrate loss from the farm is relatively low - because of low fertiliser use and minimal soil disturbance - Bichan said monthly monitoring shows the wetland's working.

"We know that it's taking out somewhere between half and three quarters of a ton of nitrate out of the water every year."

Kaiwaiwai Dairies milks 900 cows off a 350 hectare milking platform. Clay and iron pans under the paddocks mean water drains sideways rather than leaching downwards.

"If it ends up in a drain we can put it through a wetland and scrub it or polish it. And that's what this was basically demonstrated to do.

"Once the temperature's above about 10 or 11 degrees, we're taking out 95 to 99 percent of the nitrate that comes through."

In winter it still takes half the nitrate out, Bichan added.

"So it is quite temperature dependent for the processes for the bacteria. But that's nice clean water."

Bichan lifts the lid on a drum at the inlet to the wetland showing where a large kōura - freshwater crayfish - had earlier taken up residence.

Tuna - eels - also live here and the wetland's designed with wide enough bays for its resident marine wildlife to take shelter when it needs to be cleaned…

Go to this episode on rnz.co.nz for more details