Martin Place siege victim Katrina Dawson was struck by a police bullet when officers became involved in a shoot-out with gunman Man Haron Monis in the final stages of the Lindt Cafe stand-off, the police investigation has revealed.

The circumstances surrounding the 16-hour Martin Place siege, which began on December 15, is still the subject of a critical incident investigation being run by the homicide squad.

The site of the Sydney siege at Martin Place. Credit:Getty Images

However, multiple police sources have told Fairfax Media that Ms Dawson, 38, was struck by police fire that was not a direct shot and possibly a ricochet, when they stormed the cafe just after 2am on December 16 after Monis executed another of the hostages, cafe manager Tori Johnson, at close range.

Ms Dawson's cause of death will be determined by the NSW Coroner once the critical incident investigation is completed.