It appears the environmental advantages of a tram are being superseded by anxiety about the cost.

And what will be the reaction when, as some predict, the wrecking ball rolls down Northbourne Avenue to clear the way for the tracks and catenary wires?

The National Capital Authority has confirmed the worst fears of some Canberrans by revealing it is concerned about a "scorched earth" policy that could leave "two kilometres of nothing" at the now graceful entrance to the national capital.

NCA chief executive Malcolm Snow said last week "Capital Metro does not get approved", in response to a question about "scorched earth".

In a subsequent attempt to belay the impact of that statement, he said "the NCA will not be blocking this project", adding his authority wanted the very highest standards of design and construction for this key project.

This is a confusing situation, and one that needs to be clarified, because Canberra looks to the authority to supervise planning and ensure the preservation of the character of the national capital.

As well, the environmental impact statement released at the weekend found a "significant risk" associated with removing the trees in Northbourne Avenue.

The latest poll on the tram project highlights the very significant level of opposition, and the government's increasingly difficult job selling the idea.

Despite the ACT government's claims of popularity, an opinion poll commissioned by the trade union movement shows most respondents oppose a light rail system. However, they were not asked specifically about the Gungahlin to Civic first stage.

About 39 per cent of respondents in the automated poll supported the project, compared with 55 per cent support in the ACT government-commissioned research in August 2014.

Which one should we believe? Indeed, could both be on the mark?

One answer to that conundrum could be in the way the question was asked or, more simply, because the community's mood has shifted. If the latter was the case, that would represent a dramatic slide in community confidence and support for a key plank of the government's strategy.

The government may well say the union-commissioned survey targeted the more Liberal areas on the south side of the city. But, so what? The government can't simultaneously claim the tram is for the benefit of the entire ACT population, but slice and dice which section of a poll it will accept. Similarly, it borders on farcical for Simon Corbell to assert the government is driven by policy, not polls, when it is so routinely commissioning them.

The unions say the polling shows support for the jobs the tram will create, but it also worries the Liberals' strident campaign against the tram has locked in party supporters.

The tram is undeniably a potent weapon for the Liberals in next year's elections where they will continue to campaign against the project on cost, not benefits. Labor should be very worried, despite its claim of disregarding polls.