New NSW building commissioner David Chandler has revealed that the likely causes of cracks and structural movements in Sydney's Mascot Towers were poor engineering design and the incompetence of a builder who "didn't know how to read construction plans".

When asked on Friday by the state inquiry into NSW's building defects for his early views on Mascot Towers, Mr Chandler said his inspection of the apartment block on Thursday - in his first week on the job – showed the original construction was badly done.