But there is no workplace agreement to build the project, and talks are at a near stalemate after more than six months of negotiations.

Unions say the consortium has repeatedly offered draft proposals considerably below industry standards for pay and working conditions.

Industry sources said the consortium disputes this, and that generous pay rates for workers working a mix of day and night shifts would be well in excess of $150,000 a year.

Pay rates in Melbourne’s construction industry - where the building unions are the nation’s strongest - are far higher than in Australia’s other capital cities.

Victoria’s Labor government, which is close to labour unions, has a massive backlog of projects it wants built - but the costs of those jobs will be affected by the outcome of these negotiations.

It will be closely watched by Treasurer Tim Pallas and the building industry.

Last month the consortium – using laws created by the Turnbull government to limit the leverage of unions on new projects – threatened to take the matter to the Fair Work Commission.

It led to unions threatening to get a Federal Court injunction to stop the consortium.

That legal action was dropped after the consortium backed down.

In 2015 the Turnbull government changed workplace laws to give employers far greater powers when negotiating “greenfields agreements” for new projects.

The changes came after controversies over generous pay deals on big projects, including Victoria’s desalination project.

Now, if bargaining is deadlocked for six months, employers can take an agreement to the Fair Work Commission for approval without the consent of a union.

The West Gate Tunnel consortium is only prepared to bargain with the Australian Workers Union, and has given it until November to sign a deal.

But four other construction unions – chiefly the Construction, Forestry, Maritime, Mining & Energy Union and the Electrical Trades Union – insist they must be part of negotiations.

The five unions appeared before Fair Work Commission deputy president Val Gostencnik in May, in a bid to force the consortium to deal with all of them.

But a recent letter shows the consortium continues to say it will negotiate “with the AWU alone”.

As well as pay, of key concern to unions is a bid by the consortium to control when building workers can take rostered days off.

The industry standard in Victoria is 26 fixed rostered days off; the consortium is offering 12 fixed days but wants to be able to decide the timing of the other 14 days off.

On big projects, construction workers often work six days a week - around 56 hours a week - so regular rostered days off are regarded as important to combat fatigue and ensure safety.

The consortium’s attempt to deal only with the AWU to strike a deal for tunnelling works is a tactic used in the past to play it off against the more militant CFMMEU.

The tactic was used in a bitter 2009 dispute at the West Gate bridge widening project - which also involved John Holland (then owned by Leighton Holdings).

AWU secretary Ben Davis, however, wants the CFMMEU and other unions in negotiations and attacked the consortium for its offer.

“John Holland and their joint venture partners have tried to jam an inferior agreement down the throats of the building unions,’’ he said. Unions would work together to ensure “working people’s rights won’t be trampled on by greedy multinational employers”.

Unions were “not interested in seeing industry standards cut and we believe the joint-venture negotiation tactics are designed to bring that about”, he said.

The consortium was contacted on Friday for comment, but by Sunday afternoon spokesman Tim Holmes said they still did not have a response.

Electrical Trades Union state secretary Troy Gray accused the consortium of engaging in "high risk ... ideological tactics" that would slash industry standards.

He warned the dispute could cost investors in the companies hundreds of millions of dollars due to project delays and industrial instability.

“The living standards of thousands of workers in the Victorian construction industry are going to decline as a direct result,’’ he said.

A spokesman for the Victorian government said the negotiations for the project were “entirely a matter for the builder”.

Under the deal negotiated by the government, the West Gate Tunnel's builders and not the government will be liable for any cost blowouts.

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