Labor allegedly used threats and inflated costs to dictate Airport Rail Link

A submission to Victoria's construction royal commission alleges the government inflated costs and threatened to sack a senior public servant to get its preferred Airport Rail Link route over the line.

Allegations published in The Age say the government added up to $4.6 billion of unnecessary tunnelling to a Maribyrnong route to make its preferred route via Sunshine more attractive.

The government declined to comment on the allegations and said it would let the royal commission investigating crime and corruption in the construction sector complete its work.

The Victorian opposition has accused Labor of "bullying and intimidation" after reported allegations that project costs were deliberately inflated and a senior public servant was threatened into supporting the government's preferred plans for the Melbourne Airport Rail Link.

An anonymous submission by a former government planning expert to Victoria's royal commission into the construction industry, first reported by The Age, reportedly said the government had chosen a route for the rail link through the suburb of Sunshine as its preferred project and intentionally undermined other routes with increased cost estimates.

To do so, the planning expert said between $3.8 billion and $4.6 billion of unnecessary tunnelling was added to an alternate Maribyrnong route to make it a less attractive option.

Shadow Minister for Transport Infrastructure Evan Mulholland blasted the government over the allegations.

"We've seen revelations that Jacinta Allan and her office allegedly bullied, threatened and intimidated senior public servants to get a preferred political outcome on the Melbourne Airport Rail," he said.

He said Premier Ben Carroll would have been in cabinet at the time of the allegations, and pushed the premier to reveal his own knowledge of events.

"He needs to explain what he knew, and when he knew it," Mr Mulholland said.

The proposed Sunshine route will see a new station built at Keilor East connecting it with the airport and metro network, and falls within Mr Carroll's electorate of Niddrie.

In response to the allegations, the state government said it would not pre-empt the findings of the royal commission by commenting.

"The premier's first act was to establish the toughest royal commission into the construction industry to leave no stone unturned," a spokesperson said.

"The terms of reference are broad and expansive. Nothing is off limits."

Senior government minister Jaclyn Symes threw cold water over the allegations, casting doubt on their veracity.

"In terms of reading some of the allegations in the paper, there isn't even sources in relation to some of these claims," she said.

The expert told The Age that when he raised the issue with executives he was threatened with losing his job if he did not confirm Sunshine as the ideal route for the project.

The sacking threat allegedly came from the minister for public transport and major projects at the time, Jacinta Allan.

In a response published in The Age, a spokesperson for Ms Allan denied the allegations and said the former premier would cooperate fully with the royal commission.

Mr Mulholland said the Coalition would not divert from the plans to build the route through Sunshine if elected, as work had already begun.

"Contracted works are already underway, the route is the route, we will get on and do it," he said.

Centre for Public Integrity researcher William Partlett said today's allegations were concerning, and reflected a "culture of impunity" when it came to the government's handling of major infrastructure projects.

"This is just the issue of what's happening inside the black box of the executive," he said.

"If these allegations are true, there seemed to be an assumption in the government and the minister that this was largely something that they could do, they'd do without any oversight."

Dr Partlett said the allegations of intimidation were also concerning and reflected the weakness and under-resourcing of the Independent Broad-based Anti-corruption Commission (IBAC) at the time.

The anonymous source said they kept silent on the incident due to fears of having their career derailed, and later quit the department.

"There was a view that it was too risky to go to IBAC when there were significant concerns about what that would mean for the careers for these individuals, whistleblowers in the future," Professor Partlett said.

In September, the state government passed legislation to strengthen the powers of IBAC, as part of a campaign to restore trust in government.

But Dr Partlett said the government's pledge for transparency and accountability could see even more allegations come to light.

"The momentum has shifted ... I wouldn't be surprised if we're going to see even more allegations in the coming weeks and months," he said.

The ABC approached the Royal Commission into the Integrity of Major Public and Civil Infrastructure Construction Projects in Victoria for comment.