The Raiders' crowd statistics have dropped to an alarming low as the club goes through ones of its toughest periods and the Green Machine is on track for the lowest average attendance since moving to Canberra Stadium in 1990.

ACT Deputy Chief Minister Andrew Barr said on Tuesday the government had decided to reduce temporary seating from 45,000 to 30,000 to accommodate the Brumbies and Raiders as the two major tenants.

Cutting back seating would reduce significantly the $250 million price tag. There were also initial plans for the southern end of the stadium to be left open and clear of seating to offer views over Lake Burley Griffin and the Parliamentary Triangle.

Just 7094 fans watched the Raiders disaster against the New Zealand Warriors last weekend while the Brumbies only managed to attract 14,284 to their qualifier final against the Waikato Chiefs last month.

Mr Barr said the revamped plans likely would be 20,000 permanent seats and the ability to increase to 30,000 for big events, including rugby league and rugby union Tests.

"I'm ruling out anything over 30,000, that's far too big," Mr Barr said. :That brings down the cost significantly ... there has not been a sporting event in Canberra for more than a decade that's attracted 30,000 people. I would like us to have the problem that we sell out every match rather than have vacant seats for 30 years.

"On-field performance is a factor, but we have to get the business model right [for the Brumbies and Raiders]. There's no point building something that they can't afford to play at, or that sends them broke."

The Raiders were in danger of finishing last on the NRL ladder for just the second time in the club's history.

They had won just five games this season and their crowd average had dropped to 9221 a game, the lowest since 2004.

The Brumbies made the Super Rugby finals, but even a grand-final rematch against the Chiefs couldn't spark enough interest to get more than 20,000 fans at Canberra Stadium.

When the new stadium was built, Canberra sports fans would be offered the chance to buy a dual membership to the stadium and Manuka Oval, which would give them access to rugby league, rugby union, soccer, cricket and AFL action.

Mr Barr said the plan remained to have the stadium built by 2020, and construction to start in 2018. The government was considering a PPP financial model where a private company would take on the financial risk and there would be no public investment until the venue was built.

There was no timeline on when a design had to be finalised, but Mr Barr already had interest from international companies keen to take on the project.

Brumbies chief executive Doug Edwards said: "I think 30,000 [permanent capacity] is too high, the number we have always talked about is 24,000. But whether the economics stacks up for that, I'm not sure.

"With an enclosed stadium, I'm sure attendance would lift. You just can't keep putting Band Aids on Canberra Stadium."

Canberra Stadium was nearing the end of its lifespan and the venue would only have temporary repairs while the design of a new rectangular venue with a clear roof was finalised.

Officials were investigating plans to increase seating capacity in the future to ensure the capital did not miss out on hosting marquee events because of its small size.

One option was to have no seats at the southern end to allow views of the lake.

"We'll look at a membership for the new stadium and Manuka Oval and that's a way of generating some upfront capital," Mr Barr said. "We could look at an orientation which allowed views of the Parliamentary Triangle, and design options with flexibility."

BRUMBIES CROWDS AT CANBERRA STADIUM

2014 average: 12,690

v Reds: 13,670

v NSW: 17,016

v Stormers: 11,527

v Blues: 7129

v Chiefs: 16,159

v Sharks: 11,615

v Rebels: 11,071

v Force: 11,742

v Chiefs: 14,284 (qualifier final)

Total: 114,213

RAIDERS CROWDS AT CANBERRA STADIUM

2014 average: 9221

v Titans: 9636

v Knights: 9519

v Storm: 10,941

v Panthers: 8892

v Cowboys: 8412

v Brisbane: 8094

v Bulldogs: 10,873

v Rabbitohs: 9526

v Warriors: 7094

Total: 82,987