Retired admiral Chris Barrie has been a vocal opponent of the major project, fearing it will completely undermine the national institution.

"I guess the game is over in terms of the fact it is going to happen, but I'm afraid it won't be a memorial for very much longer," he told ABC radio on Tuesday.

"It might be a place of entertainment but in my view it will not be the place of remembrance I grew up with."

Mr Barrie said the redevelopment would turn the memorial into a theme park.

"I recall the place of remembrance I used to take visitors to as being unique around the world," he said.

"To try and turn it into a theme park or a place of entertainment, whatever words we want to use, is not right."

Mr Barrie is most concerned that in the pursuit of a "bigger and better" attraction, future generations of Australians will lose the opportunity to quietly reflect on the country's military history.

"We've got plenty of theme parks in this country, we don't need another one," he said.

The National Capital Authority (NCA) has given the nod for early works at the memorial, including the demolition of Anzac Hall.

At least 140 trees will be removed as part of the redevelopment, which will eventually be replaced by 250 native trees.

The NCA, which has published a 1200-page consultation report, received 601 submissions during its consultation process, of which just three were in support.

Key concerns were the need for the expansion, tree removal, the impacts on heritage values, and the cost.

Greens senator Jordon Steele-John described the approval process as anti-democratic and said it was abundantly clear the community did not support the proposal.

"The consultation process up to this point has been an absolute farce and an insult to the community," he said.

"The speed with which this proposal has been approved by the NCA makes it clear the concerns of the wider community have been ignored at the behest of the Morrison government, the AWM and their corporate donors."

Senator Steele-John said it was incredibly disappointing to see the project waved through and promised to fight to ensure the memorial remained a place of remembrance and not "a billboard for weapon makers".

The memorial argues the project will address constraints to the existing use of the building, improve the overall visitor and veterans' experience and maintain its significance as a national cultural institution.

Australian Associated Press