"It is the case that brittle gum like all other eucalyptus species can shed limbs and can shed limbs unexpectedly but that is going to be a problem with any native eucalyptus species," Mr Corbell told estimates hearings, questioned by the Liberals' Nicole Lawder about descriptions of the species as a "widow maker" for its habit of shedding branches.

"The advice to the government is that the risk is no greater or no worse than any other eucalyptus species," Mr Corbell said.

"It is a requirement of the National Capital Authority that the tree plantings along the Northbourne corridor be of a native species not an exotic species, so we were always going to face this issue of limbs falling. The advice is that that mannifera is a tree that can deliver the stately elegant shape that the NCA requires for the presentation of the avenue. It's well suited to our local climate, it's well suited to the soils in Northbourne Avenue itself and it meets design requirements of the NCA."

More than 100,000 brittle gum trees were already planted across Canberra, including on "high profile and iconic avenues" such as Captain Cook Crescent.

The government had planned to plant Eucalyptus rossii or scribbly gum, to replace the river gums that line Northbourne Avenue at the moment. But it changed its mind after discovering the soil was not suitable for the tree and Mr Corbell said the choice of brittle gum instead had been made in consultation with arborists, the Australian National Botanic Garden, the National Capital Authority and others.

Asked about National Capital Authority chief executive Malcolm Snow's concern that the four-year tram construction would result in two kilometres of "scorched earth", Mr Corbell said the authority wanted the ACT to test scope for a staged removal of trees. He had put that to the consortiums, asking them to "demonstrate to us how they are taking those issues into account and what they can deliver in terms of a staged removal and replacement".

Asked whether school buses would continue to use Northbourne, Mr Corbell said detailed network decisions had not been made.

The draft environmental impact statement envisages major bus changes, including replacing all trunk and express buses on Northbourne with the tram. Mr Corbell said with an interchange in Dickson, the plan was to realign buses to operate east-west, with the tram operating north-south. Interchange between bus and tram was not a problem as long as wait times were short, he said.

The government had spent $32.1 million on the $780 million project over the past two years, he said.

He was now in "very intensive meetings" with the two shortlisted consortiums.