Sydney teen, 16, stabbed in car park at Balmain supermarket

A 16-year-old boy has undergone surgery after being repeatedly stabbed in a Sydney car park, as police remain eager to speak with a man and woman who they believe could help them piece together what happened.

The boy was shopping at a Balmain supermarket with his brother before the attack at a car park on Darling Street just before 7pm on Thursday, which left him with wounds to his upper body.

He was taken to the Royal Prince Alfred Hospital where police say remains in a critical, but stable, condition following surgery.

A witness has recalled hearing the boy cry out before she approached him to find him surrounded in blood.

"I just saw some kids screaming and at first I thought they were mucking around," the woman told Nine News on Thursday night.

"Then I heard him just scream out 'I've been stabbed, I've been stabbed, help me'.

"I walked over and there was just blood all over the floor. He said 'I didn't know where he came from or who he was, he just came out of nowhere."

Inspector Gary Coffey said it's not believed the attacker and the boy knew each other.

He said police are eager to speak with the teenager once his condition improves.

"It's a pretty ferocious attack and certainly it's not something that would normally occur in this area," he told reporters at the scene.

Police also want to speak with a man and woman who may be able to assist with their investigation.

The man is described as being caucasian, about 182cm tall and with ginger hair, while there is no description of the woman.