Senior NSW frontbencher Jo Haylen has resigned as transport minister after a string of damaging revelations about her private use of government cars, in a major blow to Labor Premier Chris Minns and his bid to end a damaging union dispute that has disrupted Sydney trains services.

Ms Haylen fronted a press conference in Sydney on Tuesday revealing she had taken a second trip in a government car to the Hunter Valley wine region with her husband last year. She said she had resigned as a minister because her travel expenses were damaging the Minns government, but that she would continue to serve as the member for Summer Hill.