When the Northwest Metro opened last May, Premier Gladys Berejiklian was able to claim an impressive achievement. The new 22-kilometre rail link had come in “more than $1 billion under budget”. But making a similar claim once the next phase of the Sydney metro rail network opens looks most unlikely.

On Tuesday, the Herald revealed an internal review had put the cost of the rail line from Chatswood to Bankstown via the CBD and Sydenham at $16.8 billion – more than $4 billion over the initial estimate.

Ms Berejiklian and Treasurer Dominic Perrottet both sidestepped questions in Parliament about the blowout following the Herald’s report.

The review shows the biggest projected cost overruns are for new trains and systems, for excavating tunnels and sites for underground stations and for the extensive work required at Central and Sydenham stations.

The scale of the blowout is a major worry – it increases the total cost of the project by more than a third. To make matters worse, the highly confidential report was completed about 18 months ago – long before the last election.