More than 30 activists armed with placards and banners converged on the Merton St office about 2pm in what they described as an “impromptu visit”.

The protesters demanded the release of an independent review of the company’s environmental off-set strategy for its Maules Creek project.

But the demonstration turned violent when a protracted physical altercation broke out in the office’s doorway, prompting the police to be called.

The activists eventually retreated from the office and no arrests were made, although protesters claim to have video of an employee smashing one of their mobile phones.

Whitehaven Coal CEO Paul Flynn said it was “extremely regrettable” that employees were exposed to “violent intimidation and the very real risk of injury”.

“This senseless and misguided action undermined people’s right to legitimate and peaceful protest, which Whitehaven has consistently defended,” he said.

“The company, in consultation with NSW police, will take all necessary steps to protect the safety of its employees.”

Ecologist Phil Spark admitted the situation had got out of control, but said it was nothing more than a mild “push and shove”.

He said the activists were frustrated a report vital to determining the adequacy of Whitehaven Coal’s plan to mitigate the mine’s environmental damage had not been released.

“It is crucial that we see how the mines propose to meet the requirements of the approval, as the deadline for our court appeal is fast approaching,” he said.

“We know this company has failed to meet the federal government’s approval conditions and we suspect the independent review confirms this. Why else would Whitehaven Coal be delaying release of this report?”

Whitehaven Coal said the report had been lodged with the federal Department of the Environment.