Australian War Memorial boss Brendan Nelson says a $500 million expansion of the site will help veterans' PTSD.

Dr Nelson says a key contributor to post-traumatic stress disorder (PTSD) is a lack of proper recognition of military service.

“If we are to deal with and to prevent post-traumatic stress, [it's important] that no young serviceman or woman or their family feels that what they have done doesn't count,” Dr Nelson said.

“One of the key contributors to post-traumatic stress is the sense that your country doesn't know and doesn't care about what you did"”

The former defence minister joined Prime Minister Scott Morrison to publicly unveil plans for the Memorial's $500 million redevelopment this week.

The nine-year development includes a cavenous new underground exhibition hall to house recently acquired military items such as helicopters and jet fighters.

Dozens of prominent Australians, including a former memorial director, have signed a letter opposing the $500 million expansion.

They argue that the money would be better spent on veterans and their families.

“Not a cent of this money comes from the Veterans' Affairs budget, not a cent from the defence budget,” Dr Nelson said.

“Every story, every operation that's done by these Australians, we are proud of it and the stories will be told here at the Memorial thanks to the support and investment of the Australian Government.”

Dr Nelson is leaving the position this year, with a replacement to be announced in the coming days.