The channels are 2m apart and have to be in dead straight lines.
The clay then dries, leaving the channels open for water to drain through. It stays like that for 10 to 30 years - depending on how the paddock is managed. Heavy animals will collapse the tunnels sooner.
If they drain into tiles deeper underground, the drainage lasts even longer.
In places from Waitotara to the Manawatu Gorge and between the sandhills and the papa country the topsoils of this region have a heavy clay base.
"It's like blotting paper and the water just sits in it," Mr Wright said.
Farmers have been draining paddocks with this clay for 150 years, and Marton still has a business making old fashioned clay field tiles. Mr Wright's father, Colin, started his drainage business and still helps out, aged 83.
The first mole ploughing was done with draught horses, then traction engines. The P.W. Wright floating boom mole plough is towed behind a Cat D56 bulldozer.
The ploughs need a strong machine to pull them because the region also has patches of ironstone. It ranges from pebbles to large blocks in the topsoil.
"Every farmer knows about. It's pretty brutal stuff when you hit it. Some blocks are so heavy two men couldn't lift them," Bruce Cawood said.
He's been mole ploughing for Mr Wright for 22 years. From "an old Marton pioneer family", he leaves his home in Havelock North to work for the contracting business every summer.
Mr Wright watches weather forecasts and uses local knowledge to calculate whether the region's clay is ripe for mole ploughing. He takes core samples before starting, to make sure the clay is the right consistency.
The weather after ploughing is also important - it has to be dry enough to allow the clay to set.
This year there was a very wet winter, followed by a wet start to the spring.
"We couldn't start moling until the clay turned into plasticine, not porridge," Mr Wright said.
The window of opportunity lasted just seven weeks, then closed. That was unusual, but not unknown. Mr Wright said the same thing happened in 1995.
For the rest of the summer the mole plough has only been used where there has been enough rain. The clay has only been right in patches, mainly inland toward the ranges where there has been more rain.
The weather has affected crops, too.
"There's some pretty ordinary barley crops. It doesn't like wet feet, or extreme drought. Maize is growing pretty well if it was planted after the big wet. It loves the dry."
Farmers who were counting on mole ploughing to improve drainage are facing yet another soggy winter.
"It's quite dramatic what the weather has done this season. It's disappointing not only for us but for the farmers as well."
