KPMG considered four options - from the futuristic design unveiled in 2015 to sit atop City Hill, to less fancy options, to a "do-minimum" option, which still meant $152 million of upgrading work on the existing convention centre in Constitution Avenue.

All of the options failed the cost-benefit test, although the futuristic Fuksas design comes closest to getting over the line, with a benefit-to-cost ratio of 0.9 (90 cents for every $1). In second place is the "do minimum" option, with a benefit-cost ratio of 0.8.

The picture is more dismal than a 2015 rapid business case, which put costs at $433 million, and the cost benefit ratio at 2.40.

The KPMG report said the existing centre, built in 1989, was dated, small and inflexible with a three-day convention often forcing closure to other events for a week. Market share was falling. In 2013, the centre hosted 64 conventions; in 2015, the number had fallen to 45, and numbers were expected to fall to just 36 by 2023.

A new centre on City Hill could host 72 national conventions, six international conventions, and 245 concerts in 2023, plus other events. It could host three separate events at once, 2500 delegates in the main space, 1800 in the ballroom, as well as 18 smaller meeting rooms, and 180 delegates in the "centre for dialogue".

If the existing centre received the minimum upgrade, it could host 60 national conventions and 20 concerts, plus other events.

The convention centre is a centrepiece of the government's City to the Lake project, but was shelved by Chief Minister Andrew Barr when he removed $8 million from the coming budget to get it investment ready.

The move upset the business community, which strongly backs a new convention centre, and now an ACT parliamentary committee has launched an inquiry. The government released the 2016 KMPG report in its submission to the inquiry.

While Mr Barr has made much of the need for commonwealth funding and the lack of federal interest in the project, a submission from the National Capital Authority expressed strong support.

Authority chief executive Malcolm Snow said the City Hill site was ideal, at an apex of the national triangle. The Fuksas design was the standard the authority would expect, making a visual statement and distinguishing the building internationally.

A national convention centre would make a major contribution to the revitalisation of Constitution Avenue and reinforce its central status. It would free up the existing site for development and should be planned as an integral part of the City to the Lake and city centre plans.

"Importantly, it would also act as a significant public infrastructure catalyst to stimulate new private sector investment in the city centre," Mr Snow said.

The KPMG report has not included the cost of major roadworks, including ramping up London Circuit to the same level as Commonwealth Avenue for a City Hill convention centre. Nor has it included the cost of buying land from the casino to bring the existing convention centre up to scratch.

It concludes that the "do minimum" option of upgrading the existing centre with a bigger exhibition hall, possibly on to nearby land, a second-level banquet hall, new lifts, storage and kitchen areas, could deliver big benefits for a lower cost, and is "sound".

In its submission, the ACT government said support for a new centre was not as strong in the community as in parts of the business sector.

Funding was a core issue, with a number of projects that had to be paid for first - including light rail stage 2 and the Parkes Way upgrade. Both projects impacted on roads around City Hill and design of any City Hill building. If the tramline was to use the eastern side of London Circuit, it should be installed before work on a convention centre, "as it would seem inappropriate to open a new convention centre on to a light rail construction site", the submission said. The eastern side of London Circuit is not due for a light rail line until stage 3 of the project.