Activists protesting against a coal project have disrupted mining operations at several sites in the Gunnedah Basin, in northern New South Wales.

An anti-coal protest group said about 150 protesters had targeted four mines and a coal loader operated by Whitehaven Coal, which is planning a controversial mine at Maules Creek, near Narrabri in the state's north-west.

Opponents, including environmentalists and farmers, have said the project and associated forest-clearing pose significant environmental threats including to biodiversity and water required for agricultural use.

Four protesters chained themselves to access points to the Maules Creek project and two scaled the coal loader at the Werris Creek mine and unfurled a banner.

At the Tarrawonga mine at Boggabri a protestor climbed a tripod structure to block access to the site while, three others chained themselves together across the road.

Two protesters chained themselves to a gate at the Rocglen mine near Gunnedah and another pair chained themselves to a gate at the Gunnedah coal handling plant.

Police from Barwon Local Area Command said at least six people were arrested.

They included a 27-year-old environmentalist who chained himself to a rail line near Newcastle.

Police rescue officers had to cut the man free and he was likely to be charged with trespass.

The Australian Rail Track Corporation confirmed the protest had disrupted rail operations.

The rail line, which is used to transport coal into the Port of Newcastle for export, was blocked for more than two hours.

Protesters demand inquiry into 'flawed approval processes'

Helen War, from the Front Line Action on Coal group, said the protesters wanted the State Government to conduct an inquiry into how the Maules Creek project was given planning approval.

"They're calling for an immediate stop to work at the Maules Creek mine and for the New South Wales Government to conduct a full inquiry into the flawed approval processes," she said.

Whitehaven Managing Director Paul Flynn said the protests caused little disruption at Maules Creek, but there were delays at the company's other active mines.

"The police have been called for each of these activities," he said.

"The reality of it is, is that it will only be a marginal impact on us for the day, and unfortunately again another widespread consumption of important valuable community resources such as the police and police rescue.

"I’m not quite sure what these people thing they’re doing by targeting just innocent people trying to do their job."

Greens leader Christine Milne visited an activist camp site near Maules Creek on Sunday to show support for the ongoing protest action.

The party has controversially supported those involved in the unrest, including activist Jonathon Moylan.

Moylan was given a 20-month prison sentence in July after pleading guilty to issuing a fake press release that temporarily wiped millions of dollars from the company's books.

Ms Milne said Moylan was supporting the Maules Creek farming community and has had his day in court.

"And that is what you do when you're an activist but the company hasn't faced the full face of the law and that is what is wrong here," she said.

"You've got community activists who are prepared to take the consequences but the company who gets out of taking the consequences."