A Melbourne man says a stranger in a BMW waved hello before shooting him in a supermarket car park, in what police describe as a troubling, random attack.

The 30-year-old victim was shot in the thigh at about 10pm (AEDT) on Wednesday in the Safeway supermarket car park at High Street, Lalor, in Melbourne's north, Detective Sergeant Brett Kahan of the armed crime task force said.

"The victim believes that the gunman has waved towards him and he's offered a wave back," Det Sgt Kahan told journalists on Thursday.

The gunman, in a black BMW, then pulled up metres away from the victim and asked him what his problem was before firing one shot, hitting the victim below the groin, he said.

The two men did not know each other and there was nothing inflammatory said by the victim who was shot at near point-blank range, Det Sgt Kahan said.

"It's without motive, it's unexpected. The victim was not able to defend himself in any way.

"I'd describe it as rare, troubling (and) concerning for the area."

The four young children of the victim's cousin were in a car just metres away and saw the attack, while his cousin was inside the supermarket.

The victim was not known to police and his family was stunned and traumatised by the shooting, Det Sgt Kahan said.

He said the car park was lit and open, with witnesses only metres from the scene of the shooting.

The wounded man underwent surgery in the Northern Hospital on Wednesday night and is in a serious but stable condition.

The gunman is described as being between 25 and 35 years old, with dark hair.

Police are urging witnesses to the attack to come forward.