Koutsantonis confirmed that the Malinauskas Government would be “proceeding with a tunnels design solution” between the River Torrens and Darlington, with construction on the mammoth state and federal-funded South Road project to end by 2031.

The cost of the revised plans, construction start date and number of homes set to be razed as part of the 10.5-kilometre upgrade will be announced at a press conference later today.

Twin tunnels, “lowered and ground-level motorways” and overpasses and underpasses featured in the former Marshall Government’s plans, but Infrastructure SA warned the design had “major issues and/or risks”, prompting Koutsantonis to delay the upgrade and commission a review.

The review included a financial audit of the final design costings by independent consulting firm PwC and detailed cost estimates for two alternative roadway solutions as a benchmark comparison.

According to the government, the review found that the original plans would have cost $15.15 billion – $14.3 billion of which would be spent realising the reference design and $850 million for “essential” network upgrades.

That compares with the $9.948 billion price tag announced by the Marshall Government in its final business case.

An Infrastructure Australia analysis of the previous government’s business case published in February found that the upgrade proposal had a benefit-cost ratio of 0.7. Any number below one means that the costs outweigh the benefits.

Inside one of the North-South Corridor tunnels. Render: SA Government

The review found that if the government had spent $9.948 billion, only the southern tunnel section of the original 2021 reference design could have been built, with integration to South Road “at or around Richmond Road or James Congdon Drive”.

The government said that under that model, South Road would have been retained in its current configuration between the River Torrens and Richmond Road, with various intersection upgrades incorporated into the design.

It said the review found that this solution would still have costed more than the original budget, coming in at $10.9 billion.

The review also found that the 2021 business case underestimated the number of properties required for compulsory acquisition.

It found 412 properties would need to have been acquired if the former plans were realised – up from the 393 properties previously announced.

Koutsantonis said keeping a “piecemeal approach” to save money in the short-term would have prolonged uncertainty for western suburbs residents and commuters.

“It is vital we do this crucial project right first-time, and for the long term,” he said.

“Delivering a project of this magnitude required a rigorous financial assessment, and it is clear from the state Labor Government’s thorough review that the project scope included in the 2021 business case was simply unable to be delivered for $9.9 billion.

“This project is too important to risk on an ill-considered or second-rate solution – we only have one chance to do this right, and that is why we have taken the time to ensure we have the right option, correctly costed.”

The Opposition’s infrastructure spokesperson Vincent Tarzia told ABC Radio Adelaide this morning that the Marshall Government had its plans verified by independent bodies such as Infrastructure South Australia and Australia.

He questioned whether the Malinauskas Government had done the same.

“We made sure that when these plans were formulated… that we minimised the impact on residents and businesses,” he said.

“Look at how the Labor Government have released this announcement – they’ve done it outside the parliamentary sitting week when they know that they’ll be avoiding parliamentary scrutiny and they’ve done it somewhat about 10 days before Christmas.”

InDaily asked when the major works were scheduled to start. A state government spokesperson said that information would be announced at today’s press conference.

It comes after Department for Infrastructure and Transport CEO John Whelan last month told a parliament committee that the previous timeline for the North-South Corridor was unrealistic, but maintained it would be finished in 2031.

“Previously this project was presumed to have been going to market in April of this year,” Whelan told the committee.

“We are not in a position to go to market yet, and we never were in a position to go to market in April of this year.

“So as far as any delays, I think it would be remiss to say there are delays to the project – what it has been is we’ve had to relook at the old reference design and investigate some options which were of some concern.”