Sydney's light rail line is set to blow its budget again after it was confirmed that the state's transport agency is preparing a final revised cost for the botched project.

A day before an inquiry into the project resumes, a report by NSW Auditor-General Margaret Crawford into the state's transport agencies said "additional costs to the project" were expected after a $207 million contingency fund was exhausted and attempts made to resolve claims by the contractor.

"[Transport for NSW] are in the process of preparing a revised forecasted final cost," her report said.

The contingency fund was included in the $2.1 billion budget, Transport for NSW said. But the agency declined to say when it would release the revised final cost.