Conservation group the Bob Brown Foundation says it is taking "urgent legal advice" about a lease granted to MMG along an access road on Tasmania's west coast near where the mining company wants to develop a heavy metals tailings dam.
Resources Minister Guy Barnett used his discretion to grant the new lease last Monday and announced the decision on Friday afternoon.
It is the second time Mr Barnett has granted the Chinese-owned company, which owns a metals mine at Rosebery, a lease along Helilog Road.
Mr Barnett previously admitted he granted the first lease without having the power to do so.
He said the new application was assessed by Mineral Resources Tasmania, and the state's mines director recommended it be approved.
He also said the State Growth Department reviewed MMG's application.
"In light of this, I have found no compelling reason not to grant this lease application," Mr Barnett said.
"The [mines] director advised that the lease would enable the land to be used to enable the applicant to access its existing lease at South Marionoak and that it would also enable work associated with mining at the South Marionoak site to be carried out."
South Marionoak, across the Pieman River from the Rosebery mine, is where MMG has proposed to build a tailings dam.
The new application and subsequent approval have not allayed the concerns of the Bob Brown Foundation.
"We're taking urgent legal advice around the questions that this lease raises, and given the form previously where the lease wasn't lawful, we're uncertain how the new issue can be lawful," the foundation's takayna/Tarkine campaigner, Scott Jordan, said.
Mr Jordan said the lease was not needed for the project.
"They already have a road, they have a road use agreement to use that road … this lease effectively blocks people out of 7 kilometres of road ahead of the mine lease they currently have. It's designed to prevent media, it's designed to prevent the public, and particularly designed to keep protesters out of MMG's way," he said.
Mr Jordan said protesters would "not be bullied by the minister's behaviour".
"Our protest remains in place. The people coming on site now understand that the minister intends to have them charged with trespass and removed from the site. People will continue to take that risk," he said.
"This is us taking a stand against last-century practices."
Mr Jordan said the Rosebery mine had options to move to "21st-century solutions like paste fill", referring to a method used to dispose of mine tailings underground.
"Instead, it's opting to pump its tailings across the Pieman River, dump them in beautiful pristine rainforest and melaleuca forest," he said.
"This is last-century technology, it's technology we know doesn't work — there's not a tailings dam in Tasmania that doesn't leak and put acid into the environment."
Mine to close 'in coming years' without new tailings dam, manager says
MMG has started another stage of site investigations to determine the feasibility of a new tailings dam at South Marionoak. The works do not include construction of the dam.
"Without additional capacity to safely store tailings, MMG will have no alternative but to ramp down operations and close the mine in coming years," Rosebery mine's general manager, Steve Scott, said last month.
"We respect the right to peaceful protest and for people to express their opinions, and we likewise expect people to allow the completion of our approved investigations.
The company said it would take several measures to avoid significant impacts on threatened species and communities, including wedge-tailed eagles and Tasmanian devils.
