At least 10 per cent of the forests were struggling, executive director Stephen Alegria said.

A soon-to-be-completed review into the health of the trees would help staff make the tough decisions and plan for the future.

"There was a set of circumstances that caused many of the trees to fail … now we're really grappling with what should we do," Mr Alegria said.

"Should we remove the remaining trees and try to establish a new forest at that site? Should we re-establish a new forest at a different site?

"We have to think about what species would we replace those species with, how much would it cost, if we're going to use a different species what would it be and what value would it have for conservation."

Mr Alegria said the arboretum had replanted close to 1000 failed trees last year and was developing an app for staff to record the history, condition and action taken for each tree.

Although tree health was the No. 1 priority, he said staff were unsurprised at the failure rate as the arboretum had been designed to be experimental with its "botanically significant" and rare species making it of unique value.

"At the time they were carefully chosen on known characteristics and compatibility with the local conditions here," Mr Alegria said.

"It wasn't possible to determine if they would be suitable in the long term until we planted them."

But veteran forester Peter Marshall said more than 20 forests were dying and he blamed it on poor management, lack of planning and overplanting, saying the entire arboretum will be dead within 10 years unless drastic action was taken.

"The spacing is unbelievable, normal spacing is 10 to 12 metres for trees, up to 30 metres for some species, in the arboretum most trees are two to three metres apart," he said.

"It's absolutely inevitable the trees on the outside of each forest are getting more sun and they're bigger than the inside ones."

Mr Marshall, who has 40 years' experience as a forester and worked on the site when it was a pine plantation, said many of the trees were planted too densely, under-pruned, poorly irrigated and irreparably damaged by mowers and left to die.

He also criticised the site preparation saying the root structures of many trees ,including the Mesa oaks, had been compromised when they were forced into hard, stiff soil making them prone to waterlogging, heat stress and falling over and a potential danger to visitors as they grow larger.

Mr Alegria said the site was cleared before his time with the arboretum, but he was confident the trees had been planted correctly.

Mr Marshall also criticised the single species block forest layout saying it ignored tree growth habits making the arboretum doomed to fail costing Canberrans millions to fix.

He said the sheer number of trees meant staff were failing to prune out flaws with about 48,000 trees over the 250-hectare site – a rate of 192 trees a hectare when the usual ratio was 20 to 30 a hectare.

"Every other arboretum in the world plants wide and form prunes," he said.

"I'm not critical of the concept, I'm critical of the execution and the management."

But Mr Alegria said the arboretum was carefully planned and unlike traditional arboretums the Canberra site was deliberately planted with single block species in forestry-like grids and some forests were designed to be left unpruned.

Mr Marshall said the arboretum's management had failed to learn from the mistakes of existing pre-war forests on the site which had been deliberately over planted with the intention of thinning out the weaker trees – something which never occurred.

The arboretum's patron Jon Stanhope was concerned about two struggling forests including the Buchan blue wattles, when he was appointed to the position in January.

He said at the time he hoped the review into the health of the trees was released publicly, but Mr Alegria said the decision would be up to the government.