Detailed business cases revealing the financial justification for spending billions of dollars on Sydney's major transport projects remain secret after the state government refused to release them.

Transport for NSW has rejected a request from Labor under freedom-of-information laws to make public the full business cases for the second stage of Sydney's $20 billion-plus metro rail project, the $2.1 billion Sydney CBD light rail and the Parramatta light rail.

Labor transport spokeswoman Jodi McKay said the public expected the government to release documents that revealed the financial justification for large projects.

"We are talking about billions of dollars worth of projects and all we have is the release of glossy documents," she said.