The National Capital Authority, tasked with granting the project's final approval, gave it the green light for early works on Monday morning, noting it was consistent with the National Capital Plan.

Early works include excavation at the front of the institution, the relocation of services, temporary hoardings, tree removals and the demolition of Anzac Hall to make way for new gallery space.

The capital authority's chair Terry Weber said 455 trees around the grounds would be retained, while 140 trees are slated for removal as part of the project.

"The NCA will require that the AWM plant a minimum of an additional 250 native trees as a condition of approval for these works," Mr Weber said in a statement.

"With this change noted, the NCA has concluded the proposal is not inconsistent with the National Capital Plan and the works have been approved."

The memorial will be required to submit a full landscape plan, with details on the species and location of the new trees, by next year.

"We are very pleased to receive approval to proceed with the Early Works package at the Australian War Memorial for the Development Project," the memorial said in a statement.

"This is a welcome milestone and we look forward to further community consultation through the National Capital Authority process for the major works designs to be released in the coming months."

Long-time opponents of the proposal, interest group Heritage Guardians, expressed their disappointment following the announcement, with convener David Stephens arguing the plan was never for the Australian people.

"This is a terribly disappointing outcome, not just for the many people who have argued strongly against the project but also for people who value Canberra as the national capital," Mr Stephens said.

"The redevelopment has been conceived in backrooms, has avoided normal accountability processes, and has been justified by the memorial's dodgy surveys of public opinion.

"Canberra - and Australia - deserved better than this."

The announcement of the project's approval follows a public consultation period in April undertaken by the authority to canvass public concerns.

The overwhelming majority of the submissions, which have since been made public, slammed the proposal, noting the heritage value of the hall along with the felling of native trees.

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Memorial director Matt Anderson earlier told The Canberra Times the planned demolition and replacement of Anzac Hall would grow the institution's heritage value by creating more space to tell Australia's military history.

"The heritage value of Anzac Hall is the stories that are told within it, not the blade wall, and the roof line," he said in April, referring to the building's signature design features.

"If we're doubling the space available to tell the stories, we're doubling the heritage value of that building, and of the memorial's power to tell the story of continuing service and sacrifice.

"We're just not doing that, we're not meeting that obligation to be fair to contemporary veterans, modern veterans, in the same way we've told the stories of their forebears."

Mr Anderson said he would continue to work with the authority to address any issues in a Senate estimates hearing last week, acknowledging more public consultation would be needed before the main works could begin.

"We are bound by whatever directions the National Capital Authority place upon us. We look forward to working with them. We look forward to continuing to engage [with them]," Mr Anderson said.

"[For the main design package] I imagine there'll be just as many submissions when we - if we - get through this stage."

Program director Wayne Hitches added four tenders had gone to market for the larger project with three suppliers being identified as the preferred contractor.

Early works are expected to commence by the end of the month.

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