The company behind the New Acland coal mine on Queensland's Darling Downs is silencing nearby farmers with confidentiality clauses in signed agreements, an environmentalist says.

The Oakey Coal Action Alliance (OCAA) is using the Queensland Land Court to challenge an expansion of the mine to increase annual output, from 4.8 million to 7.5 million tonnes of thermal coal, across three new open-cut pits.

New Hope Coal plans to expand the mine onto 3,668 hectares of agricultural land — before it is expected to run out of coal this year.

OCAA secretary Paul King said New Hope had hired a consultant to negotiate "make good arrangements" to protect landholders surrounding the proposed stage three expansion.

"Those make good agreements they're insisting [on] have confidentiality clauses in them and we don't think that's fair," he said.

"It stops people from discussing it with their friends and neighbours, and it stops people from getting together and working out overall water loss in that district."

Mr King said there was no legal requirement for confidentiality in the agreements.

"We think that's a bit suspicious," he said.

"We need to have things transparent so everybody knows if their water table is being affected, because it goes beyond just the current landholders — it goes to the future of farming in the district.

"It's not fair, it's not transparent, and if they're doing the right thing then why do you have to keep it a secret."

Deals causing social problems in Acland community

Mr King said the agreements were also causing problems in social situations.

"Some people will start to talk about the mine and its effects and others will just simply clam up and become nervous and won't be able to talk at all," he said.

"It cuts the conversation short and it makes people unable to express what they really feel.

"If everyone's getting a fair deal then it should be out in the open.

"If they're [the company] trying to pull a swifty, then the best way to do that is to keep it confidential and with those agreements, you'd never know."

Mr King warned landholders that New Acland stage three may never proceed.

"It would be unwise, in our opinion, to enter into one of these agreements — we're advising great caution," he said.

"If New Acland coal can get these agreements all signed up then that's evidence that people are happy, and they may not be happy."

He said the company had engaged experienced consultants to work with farmers who had no legal experience.

"It's a first time runner against an experienced field, and that's an imbalance," he said.

New Hope Coal has declined to comment while the matter is before the court.