GRIDLOCK will soon consume Australia's major cities and drain billions of dollars from the economy unless the nation embarks on critical -infrastructure investment, a leading think tank says. 
Australia is already forgoing billions of dollars in potential productivity gains after investing too little in key projects, the Committee for Economic Development of Australia (CEDA) said.
And the damage bill will blow out to more than $50 billion a year in 15 years if crucial infrastructure projects in the major cities do not go ahead, the committee said.
In a report published less than three weeks before the federal election, the business-backed think tank noted the four biggest capitals - Sydney, Melbourne, Brisbane and Perth - account for about 80 per cent of the national economy, measured by gross domestic product. They will collectively need to accommodate another 5.9 million people by 2031, the report says.
"For the prosperity of the entire country it is increasingly important the nation's major cities work efficiently," it says.
"Australia has lacked -appropriate levels of investment in infrastructure for decades while the nation's population has grown at a significant pace. Without action the cost of congestion could rise to more than $50 billion annually by 2031, with the demand for key urban rail and road corridors projected to exceed current -capacity by 2031." CEDA is pushing for governments to publish long-term plans for infrastructure development and to prioritise funding for projects that deliver the biggest overall social benefit.
The process "at the political level has been less transparent than desirable", the group's -report says.
With better planning, the private sector could make a bigger contribution to infrastructure development, it says.
"(Private input) has also been under-utilised, partly -because of lack of clarity and stability in long-term planning by public authorities; there is also unused potential to add to the range of mechanisms for beneficiaries to contribute to funding," the report says.
The committee also says governments had held back from major infrastructure projects due to "perceived budget constraints" and confusion -between investment in new -infrastructure and the cost of servicing infrastructure.The report also calls for a funding shake-up, saying the nation needs more tollways, but not at "additional impost on road users".