Armed man shot dead by police officer in Sydney

An armed man has been shot dead by police at the scene of an alleged domestic violence attack in Sydney's southwest.

A critical incident investigation is underway after the 29-year-old was fatally shot in the chest in William Street at Yagoona shortly before 9am on Sunday.

Police say officers responding to reports of a domestic violence-related incident were threatened by the man, who was armed with two knives.

A Taser was deployed before a police officer twice discharged his firearm.

Officers rendered first aid before the man was treated at the scene by ambulance paramedics, but he later died in Liverpool Hospital.

Assistant Commissioner Stuart Smith said responders were faced with what could only be described as a violent confrontation and one that would have been "a very frightening event for both police and anyone who would have witnessed it".

The address where the drama unfolded had been the subject of an enforceable apprehended violence order relative to the man, he told reporters.

There were "significant entries" relative to the location, he added, referring to police records.

Other people were present during the incident and officers were working to gather witness statements.

"We are very early in the investigation so far (but) it is, without a doubt, frightening," Mr Smith said.

"The individual tried to slash and stab the female officer. She lost her footing, she fell backwards, he continued the attack.

"His focus then was momentarily drawn to the male officer, who then deployed his service firearm."

A critical incident team comprised of Homicide Squad investigators will examine the circumstances surrounding the incident, including the discharge of the sidearm.

That investigation will be subject to independent review.