Family, pets dead in Sydney triple-killing

A man arrested after a triple-killing in Sydney remains in hospital in a critical condition as police examine a St Johns Park unit for clues.

Blood splatters, two bodies on a bed and another on the floor, pets slaughtered; this was the scene that greeted Sydney police responding to an emergency call on Sunday.

A 44-year-old man was found alive, with stab wounds, inside the unit at St Johns Park, in Sydney's west. He remains in a critical condition in Liverpool Hospital.

It's understood the three bodies, all found in a bedroom, were the parents and brother of the injured man.

Fairfield Local Area Commander Peter Lennon said when the officers entered the ground floor unit in Melbourne Road they say saw a lot of blood on the floor.

"They saw two (dead) dogs, they also saw three deceased persons," he told reporters.

Supt Lennon would not identify the three dead people but confirmed they were two men and a woman.

"We are still trying to establish the actual identification of these people," he said.

Police have no motive and no weapon so far, and cannot say how the three victims died.

"We are conducting our inquiries as to what took place, we are canvassing the neighbours and the nearby area," Supt Lennon told reporters.

It is understood the man under arrest has stab wounds to the abdomen.

The triple zero call was made by an unknown male.

As other investigators begin to forensically examine the unit, the first responding officers would be counselled, Supt Lennon said.

"I've just spoken to some of the uniformed police who are upset, but they're getting on with their job as professionally as they can," he told reporters.

The incident has shocked St Johns Park locals, including one longterm resident who did not want to be named.

She told AAP she was doing her ironing when the news broke and she heard helicopters circling overhead.

"We were watching the Olympics and it came on TV," she said.

"I've been here for 24 years and we've never had anything like this."

Fairfield police and the State Crime Command homicide squad will lead the investigation.