Developing more new suburbs on Canberra's outskirts will weaken the case for light rail, planners warn.
The ACT Government says affordable housing is driving the need for greenfield sites at Gungahlin and Molonglo.
Others believe higher residential densities in town centres will provide affordable housing and essential critical mass for light rail, while safeguarding schools, libraries and other services against closures.
The Australian Institute of Architects ACT Chapter backs the Government's new push for a $1billion light-rail system.
But chapter president David Flannery said, ''It would be wrong for the Government to continue to release land at the edges of Canberra and expect a light-rail system serving Civic, the airport and town centres to work.
''Not everyone realises how the issues we face in Canberra are all linked: shopping centres and schools closing in our inner suburbs, the failure of Canberra's existing public transport, and Canberrans finding themselves spending longer and longer time in their cars each day driving to work these are all symptoms of the city's expanding footprint.''
The ACT's acclaimed urban designer, Colin Stewart, said climate change and rising petrol prices meant the Canberra Spatial Plan, which outlines the strategic direction to manage change, was no longer as relevant for sustainable cities.
He said that, instead, Canberra should be heading in a new direction, set by the likes of the director of Sustainability at Curtin University and a member of the Federal Government's Infrastructure Australia board, Professor Peter Newman.
Professor Newman coined the term ''automobile dependence'' to describe the creation of cities where people were forced to drive everywhere.
He had written books on climate change with a focus on cities, developed strategies for transport and should be a leading voice in a debate in Canberra about higher densities and a ''walkable'' city.
Mr Stewart backs the ACT Planning and Land Authority's drive for higher population densities and urban renewal in East Lake, which would over time build a compelling case for light rail.
Both Mr Stewart and the Australian Institute of Architects believe light rail's first stage should run from Civic along Northbourne Avenue to Watson and Exhibition Park.
ACTPLA's design concept report for Belconnen Town Centre's public transport and transit upgrade, issued in March, identifies four sites for 10-storey and 18-storey mixed-use developments, including more than 400 units on unleased land between Lathlain Street and Benjamin Way.
The report says, ''With the proposed release of a number of sites which will include residential uses, there is a significant opportunity to provide accessible and affordable housing within the town centre.''
The concept report supports Transit Oriented Development, which creates compact, walkable communities centred on high-quality public transport systems.
''This makes it possible to reduce dependence on private car use for journeys to work and access to retail, education, recreation, cultural and other activities.''
Mr Stewart said existing suburbs, which were losing population numbers, could have higher densities on their edges, without disturbing their amenity.
The Belconnen concept report says all suburbs (except Bruce and the Belconnen town centre which will undergo some residential development) are forecast to experience population decline as the suburbs age.