The full business case for the tram line from Gungahlin to the city, which was released last week, says the government should retain the option of a lump sum payment of 50 per cent of project debt to the consortium contracted to build and operate the line.

The amount would be paid at the end of construction in 2019, or at the first refinancing point, between two and four years after the tram begins operation.

The business case also suggests that tram fares are not anticipated to be a big money-spinner. The tram is expected to charge an average $1.01 per trip (in 2014 dollars), amounting to just $5.5 million from fares in its first full year of operations (also in 2014 dollars).

The figures assume 15,120 passengers a day in 2021, rising to 20,207 in 2031. The $1.01 is based on an average fare of $1.35, which is in line with the ACTION bus fare average of $1.33, minus 25 per cent to account for the possibility of free interchanges between bus and tram.

Capital Metro Minister Simon Corbell said the business case was intentionally conservative and "not aggressive in its assumptions".

The business case says in initial market soundings, none of the potential private partners was prepared to take on the patronage risk.

The government will wear the full cost if passenger numbers are lower than expected, although Keolis Downer, which runs the Gold Coast and Melbourne trams and heads a consortium planning to bid also for the Canberra tram, has pointed to a likely incentive payment for increasing patronage.

The likelihood of a lump sum payment to the winning consortium was also raised last week by Keolis Downer.

Mr Corbell said the government was investigating the option of making a capital contribution, which would not be paid until "a point post construction after construction risk has passed".

The decision would be "based on the best outcome for the territory".

The 12-kilometre link is expected to cost $783 million, a cost taken on by the private consortium. The government will pay the consortium an annual fee but has refused to release its estimates of how much that will be.

The only indication of this figure in the business case is an estimate of the net present cost of the annual fee, called an "availability payment", of $804 million. That implies a payment of perhaps $90 million a year over 20 years.

A spokesman for Mr Corbell said if a lump sum was paid to the consortium after construction or early in operations, that would reduce the amount paid by the government each year in the annual fee.

The Capital Metro Agency is recommending the government retain a pay down as an option rather than a certainty.