Perth's state government is planning to build a public transport system around light rails and rapid transit buses which will cope with the public transport needs over the next 20 years.

Growing demand for public transport has Perth looking for new forms of mass transport, according to Transport Minister Troy Buswell.

"Demand for public transport will double over the next 20 years, which is interesting given that population growth will be about 30 per cent," he told ABC Radio on Thursday.

"We need to be able to move more people around quickly, independent of the bumps and grinds that happen on our roads, in other words general congestion.

"Light rail can do that and bus rapid transit can do that."

Mr. Boswell didn't give an exact amount for the proposal saying it was too early to give an estimate but that government had already set aside money for three projects as part of the 20-year blueprint.

The proposal also includes an extension of heavy rail north to Yanchep, a light rail network starting with the central north corridor to Mirrabooka and rapid bus transit system along the northeaster corridor to Ellenbrook.

The areas of focus where chosen because they had the most concentration of commuters and a lot of road congestion and bus movement. Government planners are also looking for a way to route a rail line from the city to the airport.

The proposed plan is merely a framework, added Mr. Buswell with the framework driving decisions about timelines and budgets.

The State Opposition has an alternative transport plan for the city that it unveiled yesterday. The plan featured a circle rail line diverting around the city centre to connect with growing employment zones around Perth like Curtin University, Stirling and the Perth Airport. The proposed circle rail line will be built over the next 30 to 50 years.

Opposition Leader Eric Ripper agrees that the heavy rail line is a more expensive option.

"My instinct is it's more expensive than light rail but it's more effective because it moves people more quickly and therefore you've got more incentive for them to get out of their cars and get onto public transport," he said.