The NSW treasurer has stepped up to "take full responsibility" for a cost blowout in the CBD-eastern suburbs light rail link, after an audit revealed poor planning and omissions in the original business case for the project had cost taxpayers an extra half a billion dollars.

The anticipated budget for the CBD and South East Light Rail project jumped from $1.6 billion in November 2013, when the business case was published, to $2.1 billion when Transport for NSW (TfNSW) signed the main works public-private partnership contract just over a year later. Then-transport minister Gladys Berejiklian came…