The government had planned to remove the 450 trees in stages over a year, with the third tranche to come down at the end of the year.

But Transport Minister Meegan Fitzharris said the trees were a risk to the safety of workers building the light rail line and to the wider public.

Arborists had discovered rot and fungal infestations in a number of the trees that had been felled, and other trees had smaller and shallower root balls than expected.

"Recent damage sustained from storms and the deteriorating health of a number of remaining trees has led to the decision to accelerate the final phase of tree removals," she said.

"We have always said that safety is the number one priority of the project, and that's why it's necessary that we carry out the final phase of tree removals from the Northbourne Avenue median over the next six weeks."

The stretch of Northbourne Avenue from Alinga Street in the city to Antill Street in Dickson had about 450 trees, with about two-thirds removed to date. The remaining one-third would go by the end of March.

Ms Fitzharris said the National Capital Authority had reviewed the state of the trees in early February and agreed they should be removed for the safety of workers and the public.

University of Canberra landscape architect Andrew MacKenzie, who has practised as an arborist, said as soon as some of the trees were removed the risk escalated that others would fail - so it had been an optimistic idea to stage the removal over a year.

In addition, river peppermint trees were notorious for having shallow roots.

"When you have trees that are growing together they collectively buffer against the wind so as soon as you start taking out trees separately they do pose a much greater risk," he said.

The trees had been well irrigated and the soil had had very little compaction. But irrigation was now turned off and construction trucks and other machinery was working on the median strip, compacting and moving the soil. Digging and other work would further undermine the trees.

"I suspect that their original approach was try and not clear fell just to kind of ease us it into it, but from an arboricultural perspective, it was always going to be the case that they would have to come out pretty quickly once they started removing them," he said.

"When you're taking such large numbers out and the soil's getting hammered by all the construction work, it's just a foregone conclusion. They were optimistic to think they could leave trees in as long as they planned to. Which is a pity but it's also the reality."

Rot was a lesser issue, he said. "If you applied the 'it's got rot' approach to the rest of the trees around the city we'd have a desert storm approach. If you look for it, you'll find it."

Dr McKenzie said the state of the trees highlighted the need for serious attention to regeneration of the soil for replacement trees.

"How that area is prepped for new trees is critical and we're yet to find any information about how they're going to do it," he said. "The risk always is they get to the end of the project, the budget's blown out, the tram's in and big sigh of relief, it's all finished. [But] those other bits, the landscape itself, really needs to be treated seriously."

The replacement tree canopies must do the same job as the current broad canopy to shade the light-rail centre track and the roads either side.

Plant physiology expert at the Australian National University Cris Brack said it was not unexpected that if "a few go, a lot will go", if trees were from the same species and in the same area - possibly also from the same nursery and planted at the same time. In the case of Northbourne, the watering regime was unusual for the species and could have encouraged poor root formation and decay.

Trees could be made more resilient through diversity - using different species, or at least genetic diversity within single species, and planting trees at different ages, he said, but given the desire for an avenue effect and uniformity, and the need for natives on Northbourne, the options were limited, Associate-Prof Brack said.

The replacement trees are being grown at Yarralumla Nursery to a height of about 4m.

Ms Fitzharris said they would be planted from late spring or early summer.