Farmers were encouraged by Fonterra subsidiary Farm Source Waikato to attend Monday's march in Opposition leader Jacinda Ardern's home town of Morrinsville, where many voiced their opposition to Labour's plan to introduce a tax on water.

In a group email obtained by the Herald, the head of Farm Source Waikato, Paul Grave, suggested farmers and their friends and neighbours attend the gathering.

"As you may be aware, a group of Waikato farmers have organised a march in Morrinsville this Monday 18 September to highlight the environmental leadership shown by the dairy industry in our communities," Grave said.

"The march is a great opportunity for dairy farmers to show solidarity, engage with the Waikato community, and talk about the significant improvements that have been made on-farm in recent years, and plans for further work in the future," he said.

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"We hope to see as many of you there as possible," he said.

"Everyone is welcome to join the march, so feel free to encourage your friends and neighbours to join us and show their support."

The organisers said it was not a political rally.

However, among those in attendance one man held up a sign stating, "She's a pretty communist".

Ardern has often invoked her upbringing in Morrinsville, saying it had given her an appreciation of the issues facing farmers and horticulturists in New Zealand.

Labour plans to charge commercial users between 1c and 2c per 1000 litres of water.

Federated Farmers Waikato president Andrew McGiven, in a speech to those attending Monday's march, said farmers were becoming the punching bags of the election and pleaded for some understanding of the role of farmers.

"New Zealand is too small for a rural-urban divide," he said.

A Fonterra spokesman said the company was not taking a political stance by encouraging farmers to attend the march.

"Our farmers were encouraged to attend the event in Morrinsville to engage with the Waikato community, talk about the significant improvements that have been made on-farm in recent years, and plans for further work in the future," the spokesman said.

"This wasn't a political stance - in fact the co-operative and our farmers are disappointed that some attempted to take advantage of the event as an electioneering platform," he said.

Farm Source has a network of stores throughout New Zealand, offering rural supplies and technical advice.

Fonterra, New Zealand's biggest dairy co-operative, was formed in 2001 when an act of Parliament was passed to allow the three-way merger between the country's two largest dairy co-operatives, New Zealand Dairy Group and Kiwi Cooperative Dairies, and the New Zealand Dairy Board.

Opinion polls suggest this Saturday's general election will be close. Last week's 1News Colmar Brunton poll showed Labour was leading National by four points with 44 per cent to National's 40 per cent.