Ontario's police watchdog has cleared a Toronto officer in the death of a 52-year-old woman who was wielding a knife in front of her house.

The Special Investigations Unit says Sylvia Klibingaitis called 911 on Oct. 7 and told the operator she had a knife and was going to commit a crime.

The unit says two police officers responded and one approached the house just as Klibingaitis came outside with a large knife in her hand.

It says she went toward the officer in a threatening manner and refused to drop the knife when he pointed his gun at her, stepped backwards and repeatedly told her to put the knife down.

The agency says the in-car video of the incident shows the woman was less than two metres away and holding the knife above her shoulder when the officer fired three times.

The officers performed CPR but the woman died of a gunshot wound to the chest.

Ian Scott, director of the agency, said the officer was justified in using lethal force under the circumstances.

"The subject officer was attacked by Ms. Klibingaitis who was armed with a knife and would not comply with his commands to drop it," Scott said.

"While the death of Ms. Klibingaitis is a tragic event, in my view, the subject officer was justified in discharging his firearm because he had a reasonable apprehension of death or serious bodily harm with no escape."

The SIU is an arm's-length agency that investigates incidents involving police where there has been death, serious injury or allegations of sexual assault.