Greenpeace agriculture campaigner Gen Toop claimed the plan was "full of crap, just like our rivers" and the only thing that would save polluted waterways was fewer cows.
"The dairy leadership is betraying New Zealanders with their cynical PR spin," Ms Toop said.
But Federated Farmers said opening dairy farms to all New Zealanders was an opportunity for farmers to debunk some myths around farmers' environmental management.
"This is a great idea and an opportunity for all Kiwis, rural and urban, to visit a farm and see at first-hand the environmental work farmers have done and are still undertaking," said Federated Farmers dairy industry chairman Chris Lewis.
"Everyone is welcome, especially those people who have been less complimentary towards farmers. They can see what perception is versus actual reality."
Northland dairy farmers opening their gates are Grant and Danielle, who have been farming at Waimate North for six years, and Luke and Lyna, who have been on their Waiotu farm for 15 years, the first five in a 50/50 equity partnership with Lyna's parents and since then in their own right.
The surnames of the couples and their farm locations are disclosed to potential visitors who register on the Fonterra website opengates.co.nz.
Chris Lewis said the 40 farms picked for open day scrutiny met criteria around car parking and health and safety.
"The reality is you can't have dozens of people trooping across properties or paddocks. It will have an impact and so these selected farms are suited to handle that rate of activity," he said.
"We want the visitors to see that farmers are environmentally smart and committed to ongoing improvements. As a farmer, it's our day to showcase our business and the pride we have in what we do."
To attend the open gates events on December 10, visit www.opengates.co.nz, select a button which says 'Register here to visit this farm,' fill out a few details and you will be emailed the farm address.
