NZ Farmer - Water maze to filter sediment is part of a plan to protect Waikato farm's lake

At the side of a Waikato farm lake is a maze of S-bends and narrow-necked pools.

Stream water meanders through the hurdles on its way to a peat lake on Mill Ridge farm, filtering sediment along the way.

The farm was one of 40 around the country to accept visitors as part of Fonterra's Open Gates day.

Among the 163 people registered to visit the Ōhaupō Road farm were Hamilton's Allen sisters, who learned a couple of things.

"Cows have four stomachs," Hannah Allen said.

They can eat up to 70kg, sister Lucy said, and that produces about 20 litres of milk.

But Lake Rotomānuka was the focus of the day, thanks to attention from Living Water: a Fonterra and Department of Conservation collaboration.

Through Living Water, Fonterra has invested $20m in five catchment areas.

"The water used to run straight into the lake," said Stephen Fullerton, who sharemilks on the Mill Ridge farm.

He and wife Karen have been running it - alongside their own farm business - for 10 years.

Their son Conner manages it, and he and brother Ben are the fifth generation to farm in the area, Fullerton said.

Six hectares of lakeside land was retired from farming about five years ago.

The natural filter system and wetland planting have come since.

"All that work was paid for by Fonterra. I don't know what figure - it would have been hundreds of thousands of dollars."

Fullerton sees it as a win-win, because the land had become too wet for farming and Waipa District Council wanted to buy it.

Farmers have been painted as environmental villains, Fullerton said, and it frustrates him.

"You can't really do any more - we reuse our effluent, we do soil fertilisation testing every two years, we run scales on our spreaders so we know exactly how much is going on," he said.

"If you don't adhere to the rule of Fonterra, they don't pick your milk up."

About 225 cows, mostly Friesians, are milked on the 72-hectare property.