The State Opposition has promised to build a railway line to the Perth Airport by mid-2018 if it wins the election in March.

The Opposition Leader, Mark McGowan says the 10.5 kilometre rail line, stopping at Redcliffe, the airport and Forrestfield, would cost $731 million.

It does not include the cost of new rail cars to service the line.

Mr McGowan says the airport line is a priority for Labor.

"It's the fastest growing airport in Australia, it is going to have 20 million passengers per year," he said.

"They all have to get into the airport and out of the airport."

He says traffic congestion is choking the city.

"Perth needs a rail line to the airport to ease road congestion and provide better services for tourists and business travellers," he said.

"Everyone knows that gridlock is choking our city.

"It's time to do something about it."

Labor plans to build the airport line as part of its heavy rail Metronet proposal.

That includes a line to Ellenbrook, extensions to Yanchep and Byford and a number of new stations.

Labor has costed Metronet at $3.8 billion but the Treasurer, Troy Buswell, has previously estimated it at $6.4 billion.

The Premier Colin Barnett does not doubt his figure.

"I would back Troy Buswell compared to Mark McGowan," he said.

Mr Barnett says the Liberal Party will announce a fully costed transport plan as part of its election campaign.