Conservation expert Ian Fraser said existing eucalyptus elata trees on Northbourne Avenue's median strip were a poor choice for the area and should be replaced by species native to the capital region.

The government agency planning the 12-kilometre tram line has flagged a possible grass surface for the tracks and is considering a unique Canberra aesthetic for passenger stops.

Designs include a wired power supply through a single overhead line in each direction.

Mr Fraser said the current trees, also known as river peppermint, were more suited to costal areas with gullies and wet conditions.

"The only way they could get established was to give them lots of water, which is obviously not desireable in our climate," Mr Fraser said.

"By giving them lots of water early, it makes them shallow rooted. If there's lots of water near the surface, the tree has no incentive to put down a deep tap root.

"Some of them blew over very quickly, as we predicted they would, because they're not adaptive to this particular climate."

The author and former ACT Conservation Council director said Northbourne Avenue could showcase Canberra region trees after the light rail line is built and the area be improved through longer lifespan and fewer fallen limbs.

Capital Metro Agency director Emma Thomas said on Monday 202 of the existing 484 existing trees were healthy, and most would need to be replaced within five years.

The line is expected to begin operations in 2019 or 2020, surrounded by native trees.

"I'd be looking for something like yellow box or red box, possibly snow gums or a mix," Mr Fraser said. "There's no reason why an avenue has to be all one species."

"The problem in Australia of the concept of native plants is that we're an entire continent so native doesn't necessarily mean local to Canberra. A Western Australian tree is no more native to here than something from Turkey is native to Britain."

The track's surface is yet to be finalised, with safety, efficiency and aesthetic value among considerations.

Concrete, paving, granite, ballast, asphalt and grass will be considered.

Ms Thomas said ground power supply could be used in isolated areas like bridges, but it was unlikely in the city to Gungahlin route.

Planners will seek to minimise the visual impact of power lines on the area's urban landscape.

Work is continuing to determine technical requirements for tree planting along the route, considering spacing from the carriageway, platforms and other overhead and underground services.

A National Capital Authority spokeswoman said tree species would be selected after a landscape structure plan had been developed.

"This will be prepared by the ACT government, and will be part of the urban design guidelines for the Northbourne Avenue corridor," the spokeswoman said.

The actual species of tree will be decided through colour and shape, density of planting and local diversity.