Northland Fish and Game is objecting to the renewal of resource consent for Northpower's Wairua power station.
The small hydro station west of Whangarei has traditionally provided access for people wanting to get down to the river.
But Northpower has installed locked gates on the access road about two years ago, for safety reasons, it said.
Fish and Game's regional manager Rudi Hoetjes said that has made it very hard for the public to get to three conservation reserves on the Wairua River.
People increasingly want to visit those places, he said.
Northpower also lacked a plan to protect trout and native fish,including whitebait, Mr Hoetjes said.
He said trout were stranded and died every time the company drained the canal for maintainance and that consultation was needed so people could try to save the fish.
Northland's rivers campaigner Millan Ruka is also objecting to the resource consent and the Northland Regional Council has put it on hold pending a response from Northpower.
"The gates are huge, imposing, demonstrating there was a complete lock off," he told Morning Report.
"The security and health and safety stance Northpower has taken is as if it was a nuclear power station, and it's not of course, it's our 100-year-old power station."
Mr Ruka, founder of Northland's Environmental River Patrol, said the gate had his role more difficult.
In 2011 he discovered Northpower was in breach of its resource consent as it had failed to provide a fish pass at its hydrostation, as the law demanded.
Powerpower spokesman Steve MacMillan said Mr Ruka had been offered supervised visits beyond the gate, visits Mr Ruka said were "short, impractical and humiliating".
The Department of Conservation said it was unaware of the problem and that staff would visit the site next week to assess the situation.