The Berejikilian government is under renewed pressure to release the entire business case for the metro rail line it is building under central Sydney, the cost of which has blown out by up to $3 billion.

A parliamentary inquiry into the project found the "case and rationale" for converting a 13-kilometre stretch of the Bankstown Line, which will form part of the Sydney Metro City and Southwest project, has "not been adequately made out".

The government has released a summary of the final business case for the 30-kilometre rail line from Chatswood to Bankstown via the CBD, leading to accusations the project lacks transparency.

The plans to convert a section of the Bankstown Line to carry single-deck metro trains have been controversial. Credit:Simon Alekna

The inquiry's report, released on Thursday, recommended the full business case be released immediately, and that the government end the metro line at Sydenham, instead of continuing it on to Bankstown.