A stop on the Mandurah rail line described by senior Labor MPs as "a train station nobody wants" and "blatant pork-barrelling" will be fast-tracked to be built within five years as part of a major infrastructure funding announcement made this week by Prime Minister Scott Morrison.

Key points: The WA Government prioritised Karnup as the next Mandurah Line station built

The WA Government prioritised Karnup as the next Mandurah Line station built A national infrastructure announcement instead put money towards Lakelands

A national infrastructure announcement instead put money towards Lakelands The Lakelands Station is in a Liberal MP's electorate, while Karnup is in a Labor seat

The WA Government insisted it had little choice but to agree to build a new station in Lakelands after funding demands from the Commonwealth.

The Government has also abandoned a pre-election promise to open a new station in Karnup by 2022 as part of the funding agreement with the Commonwealth.

WA will contribute $16 million towards Lakelands station, five kilometres south of Karnup on the Mandurah line, which is now due to open in 2023.

The Federal Government will fund the remainder of the $80 million project, which Federal Labor MP Madeleine King said was "based on cynical pork barrelling and political point scoring".

Demands for station in Liberal MP's electorate

But Tourism Minister and Warnbro MP Paul Papalia, who described the station as a project "nobody wants" in State Parliament earlier this year, insisted the WA Government made the right call in deciding to proceed with a project it has long criticised.

"The Government is getting tens of millions of dollars from the Federal Government, who are insisting Lakelands gets built," Mr Papalia said.

"You can't knock that back on behalf of the WA taxpayer."

Karnup Station was previously a higher priority for the WA Government. ( ABC News: Jonathan Beal )

Mr Papalia said requests by the state to swap the Commonwealth funding to the Karnup station were refused and he blamed the Federal Government for prioritising a project in Liberal Andrew Hastie's seat of Canning.

He said WA would eventually build Karnup station, entirely with state funds if needed.

Mayor furious as Karnup station plans derailed

Both projects are designed to cater for population growth in the region and ease pressure on the existing Mandurah station.

The situation angered local authorities, with Rockingham Mayor Barry Sammels saying his council had been assured for years that Karnup station would be built first.

Mr Sammels had written to the State Government seeking guarantees about when the project will be built.

"We want a concrete commitment when they are going to do it," he said.

"People in that area have been let down."

Labor, Liberal MPs back different station plans

Ms King said the decision was done for purely political reasons given the proposed Karnup station would be built in her safe seat of Brand.

She also blamed the Commonwealth for the situation, but urged the WA Government to meet its promise on Karnup with or without federal funding.

"I would definitely like to see Karnup built wholly with state funds and as soon as possible," Ms King said.

Mr Hastie said the community demand for a station in Lakelands was overwhelming.

Liberal MP Andrew Hastie insists the Lakelands Station, in his electorate, is needed. ( ABC News: Robert Koenig-Luck )

"They want a train station in Lakelands and have been waiting a long time," he said.

"It's not a questions of Lakelands or Karnup, both have their place."

It is not the first Metronet promise the WA Government has failed to meet, with the timeframe of the Ellenbrook rail line delayed by a year.

Aspects of Metronet have also suffered significant cost blowouts.