Premier Gladys Berejiklian has been accused of lying about her knowledge of the real reasons for a $549 million blowout to the cost of the Sydney light rail project, but refuses to release a document that would set the record straight.

Fairfax Media on Thursday revealed that in October 2014 Ms Berejiklian, as transport minister, was sent information contained in a briefing paper from Transport for NSW about the cost increase.

Two months later, in December, Ms Berejiklian issued a statement on the blowout, attributing it to improvements in the project.

But an auditor-general's report in December last year said this was incorrect.