Capital Metro Minister Simon Corbell said on Tuesday that detailed patronage modelling was continuing for the project's business case, including consideration of extending the first stage through busy shopping areas and to Defence offices in the Russell precinct.

With services expected to commence by 2019 or 2020, the Capital Metro agency estimates 3500 passengers will travel on the Gungahlin to the city line between 7am and 9am by 2021, with 3400 passengers using peak services between 4pm and 6pm.

Daily boarding rates are estimated at 13,700 for the 12-kilometre line.

As many as 4600 passengers could board tram services in the off-peak midday period between 9am and 4pm, with a further 2200 estimated on the line between 6pm and 7am.

By 2031, total passenger numbers are estimated to have increased to 20,700 per day, with 5500 using morning peak services and 5300 the afternoon peak.

The 9am to 4pm off-peak period is estimated to grow to 6400 passengers by 2031 and 3500 passengers are expected in the off-peak period.

The figures, released by the government's Capital Metro agency earlier this month, are expected to be further refined as part of the final business case which is due to be handed to the government in October.

Modelling for the line has been completed using household travel surveys, traffic counts, MyWay ticketing data from existing bus services and participation surveys for activities including work, education, shopping and recreation.

Mr Corbell said the line's extension to Russell and along City Walk or Akuna Street remained on the cards for the project's first stage. Early plans include Northbourne Avenue, the Federal Highway and Flemington Road, with a terminus at Alinga Street.

''We will be continuing to refine these figures and they will be considered further as part of the final business case,'' Mr Corbell said.

"What we see particular in the midday table is a strong level of patronage, indeed stronger than the AM peak so that’s a useful indicator of our capacity to have a sufficient level of patronage when the project commences operation," he said.

"What it also highlights is that over time, that patronage will continue to grow."

A rapid business case for the project, leaked to The Canberra Times in May, suggested an early extension to Russell and the Parliamentary Triangle.

That document said the strongest patronage levels per kilometre would be found between the city and Dickson and that 6600 public transport trips took place within the light rail corridor.

Mr Corbell said detailed modelling was taking place for potential passenger numbers in the city and Russell.

"What we are doing is looking at what patronage would be for an extension from Alinga Street through the city to Russell and a range of possible options in terms of the alignment through that route," he said.

"It could be possible, for example, to extend the line into the heart of the CBD such as into a location somewhere, say, along City Walk or Akuna Street."

Modelling for the business case is considering travel times of 25 minutes between Gungahlin and the city in peak periods, saving commuters between one and three minutes based on 2012 average travel time figures.

With a maximum capacity of about 200 passengers each, morning peak tram services are expected to be every seven minutes in 2021, increasing to every four minutes by 2031.