UPDATE: NEW WA Premier Mark McGowan has labelled the decision to start work on the Roe 8 project at the wetlands "grossly reckless and grossly irresponsible".

His comments come after it was revealed the former State government ignored an option for starting work on Roe 8 before the election which would have saved taxpayers tens of millions of dollars and avoided irreversible damage to the Beeliar wetlands if the project was cancelled.

A leaked ministerial briefing note prepared by Main Roads for former transport minister Dean Nalder on September 2 which set out options for managing the “financial and other risks” of Labor winning the election and halting construction.

Option one was to stick to the schedule of starting clearing east of Bibra Drive in January and work west through the sensitive wetlands in February, just weeks before the March 11 election. This carried the financial risk of $50 million in “sunk costs” for contract cancellation, $5 million for initial clearing restoration and $5 million in rehabilitation costs over 20 years.

Option two was to direct the contractor to begin clearing work at Fiona Stanley Hospital by extending Murdoch Drive south ready to link to the planned extension of Roe Highway, and delaying wetlands clearing until after the election.

The sunk costs were halved in the second option and involved just $1 million each for clearing restoration and rehabilitation over the same period, although a four-week delay to the project would add $6 million.

Despite Main Roads flagging the increased cost and “perception of irreversible impact on the wetlands should the contract be cancelled” associated with option one, the Liberal-National alliance stuck to its original schedule. The document casts doubt on Liberal claims that Labor’s cancellation of Roe 8 will cost taxpayers closer to $140 million. The Government is still assessing the implications of the decision, but sources indicate it is on track for the $50 million outlined by Main Roads.

Camera Icon Protesters outside the Roe 8 site. Credit: PerthNow, Danella Bevis.

Mr McGowan says he's furious the Barnett government chose the more expensive and environmentally destructive option to start the contentious project, knowing an election loss was likely.

"They went for the far more destructive and expensive option - I'm very angry about that," he told 6PR radio on Thursday.

"They thought they were going to lose the election so they decided to burn the house down. That's cost the taxpayers of Western Australia tens of millions of dollars more."

Labor suspended work on the project within 24 hours of its landslide election win.

Mr McGowan says he expects to keep the cancellation cost at about $50 million, subject to negotiations with the companies involved.

Transport Minister Rita Saffioti accused the Liberals of reckless and deliberate indifference to the public interest. “The agency provided them with an option to minimise costs prior to the election, but they opted to ignore sensible advice,” Ms Saffioti said.

Opposition Leader Mike Nahan said Labor had not formally resolved to scrap the project until January. He claimed the Liberals had “every expectation” the road would go ahead.

“We asked the contractor what was the best thing for you to do given we asked them to build the whole road,” Dr Nahan said. “The first stage of the project was to clear before site works.”

Meanwhile, Liberal MP Dean Nalder, who quit cabinet two weeks after the main roads advice was prepared, told ABC radio he did not recall receiving it but did remember advocating for the project to begin sooner than it did.

“I was saying if you were to do it in the middle of the election campaign that would be inappropriate,” he said

Conservation Council director Piers Verstegen said many in the community would not forgive the Liberals for starting the clearing where they did.

The Conservation Council of WA has called for a Royal Commission into the project following the revelations.