It had a number of concerns about the forum, one of which was an inability to speak out on its environmental concerns.
"There's the expectation that we will work inside a process that is largely behind closed doors and not actively engage in the public area."
This had created a chilling effect on public discussion of land and water issues, he said.
Fish and Game also felt little progress was being made by the forum. Once its next report has been sent to the Government, it will have made a total of 218 recommendations on freshwater, with little tangible change in water quality.
"We are coming to the view that the Government has no implementation plan," Mr Johnson said.
"We just got frustrated with the process and thought this isn't going anywhere for us."
Another member of the forum questioned Fish and Game's exit.
Environmental Defence Society executive director Gary Taylor said any claims of muzzling were "complete rubbish".
The forum had protocols similar to Chatham House rules which prevented people using frank discussion inside the forum to their advantage in lobbying or public debate.
Mr Taylor said Fish and Game had been vocal on freshwater issues since the forum began five years ago.
The forum includes businesses, NGOs, iwi, scientists and electricity generators and aims to find collaborative solutions to managing freshwater.
Mr Taylor said the forum was making significant progress, and was close to finishing work on minimum, enforceable standards for rivers and lakes around the country.
Primary Industries Minister Nathan Guy was challenged on the issue in Parliament today.
Mr Guy said any participation in the forum required a level of good faith from participants, but this did not extend to muzzling.
He said Fish and Game's resignation "at this late stage" was disappointing.
