The West Australian Government announced plans to sink the rail line to Perth airport without official costings by Treasury, the Transport Minister has revealed.

Last week the WA Government revealed its plan to sink the entire airport rail link at a cost of $2.2 billion, which was $300 million more than forecast.

The project was a key Liberal Party promise in the lead up to the 2013 election, and the Government pledged it would cost $1.9 billion and be built by 2018, later pushed back to 2020.

Originally only four kilometres was going to be underground.

Transport Minister Dean Nalder said at the time the delay was the reason behind the $300 million blow-out.

But when addressing Parliament today, Mr Nalder revealed the new figures had never been officially costed by Treasury.

"We've analysed it, but those things haven't been put to Treasury at this point, they were put to Treasury at the last election," he said.

He said the Government then factored in an "escalation effect" to come to a figure of $2.2 billion, rather than take it back to Treasury.

Labor's Rita Saffioti went on the attack, accusing the Government of unveiling the project without knowing its true cost.

"Don't tell me you did this," she said.

"So on the weekend, you announced a $2.2 billion project without Treasury costing?"

Ms Saffioti said she was shocked by Mr Nalder's comments.

"It's a major announcement and you would think that Treasury would cost a $2.2 billion project, one of the biggest projects the Government has ever undertaken," she said.

Mr Nalder told Parliament while there was no formal costing, Treasury was "comfortable" with the $2.2 billion figure.

The airport rail link would run off the existing Perth to Midland train line near Bayswater station, tunnelling all the way to Forrestfield with stations west of the airport and at the planned consolidated terminal, which will include domestic and international terminals.