The budget for one of the WA Government's signature Metronet rail projects has blown out by more than $200 million, with the completion date also pushed out by two years.

Key points: The Thornlie-Cockburn Link budget has risen from $474 million to $716 million

The Thornlie-Cockburn Link budget has risen from $474 million to $716 million The delayed 2023 completion date has been blamed on relocating infrastructure

The delayed 2023 completion date has been blamed on relocating infrastructure The State Opposition says the Government has failed to deliver on Metronet

The Thornlie-Cockburn Link is designed to connect Transperth's southern Mandurah line to the Armadale line in Perth's east.

Before the 2017 election, Labor said the line from Thornlie to Cockburn was due to be completed by 2021 at a cost of $474 million.

But that figure has now been revised up to $716 million, something Transport Minister Rita Saffioti said was due to relocating third-party infrastructure.

The Thornlie-Cockburn Link (in light blue on the map) is just one part of the WA Government's Metronet plan. ( Supplied: WA Government )

"In relation to developing the detailed plans for this project, we've had to negotiate with other parties, in particular Arc Infrastructure and BP," she said.

"As part of this project we're relocating not only the freight line but also a major oil pipeline, so those issues have created a starting date of 2023, but we expect to finish construction in 2022."

Ms Saffioti dismissed suggestions the Government had let down residents in Perth's southern suburbs by delaying the project's completion date.

"The people in the suburbs know that this Government, together with the Federal Government, is delivering Metronet," she said.

"When you look at all the projects that are happening, people know that we're delivering our projects.

"And we're going to deliver first-class public infrastructure well ahead of any other government in the state's history."

'They have failed to deliver': Opposition

Shadow Transport Minister Libby Mettam said the blowout amounted to 51 per cent of the initial cost and was a failure by the State Government.

The new link from Thornlie Station is designed to connect the Armadale and Mandurah lines. ( ABC News: Jacob Kagi )

"Just like Yanchep and Ellenbrook rail, construction has been delayed," she said.

"This is not only of great concern for the residents of these areas who were promised a rail line in 2021 and will get something two years later.

"It's holding up valuable federal funding and holding up construction jobs as well."

Ms Mettam said the reasoning behind the blowout would not wash with the WA public.

"What we have seen by this Government since their first term is that their Metronet projects have all been delayed," she said.

"They've missed every milestone and they have failed to deliver."

Work expected to create 3,000 jobs

Federal Finance Minister Mathias Cormann was with Ms Saffioti and Premier Mark McGowan on Tuesday to announce a $1.25 billion contract had been signed for the delivery of the Thornlie-Cockburn line, as well as an extension of the Butler line to Yanchep.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 1 minute 1 second 1 m 1 s Artist's impression of new Transperth trains

NEWest Alliance would deliver both projects under a bundled contract arrangement, with the work expected to create 3,000 jobs.

The news came one week after the McGowan Government signed a 10-year, $1.3 billion contract with global train manufacturer Alstom to build 246 new rail cars to service the Metronet project.

The deal returns local rail car manufacturing to WA, following the closure of the Midland Railway Workshops in 1994.