Australia's peak infrastructure body had major doubts about the stated benefits of Sydney's light rail following a plea by the state government for $500 million in federal funding for the project, a leaked assessment reveals.

The analysis of the project by Infrastructure Australia in 2013, obtained by Fairfax Media and the ABC, raised questions about the benefits to commuters and whether the proposed light rail line would lead to an “effective increase in the capacity of Sydney's transport system”.

It cited modelling that traffic congestion in Sydney’s central business district could worsen by 12 per cent due to a light rail line, and “be worse” along Anzac Parade in the south-east.

The assessment also warned that the light rail line would not cut travel times for commuters, and that many intersections along Anzac Parade would deteriorate from “free-flowing conditions” to congested.