Former Comanchero bikie chief Mahmoud Hawi has died after being shot outside a gym in Rockdale, in Sydney's south.

Officers were called to the incident at the Fitness First gym on West Botany Street shortly after midday.

Mr Hawi was in his black luxury four-wheel drive when he was shot at least half a dozen times.

He went into cardiac arrest and was taken to nearby St George Hospital in a critical condition, where he later died.

There was a heavy police presence outside the hospital, including three riot squad vehicles and a police car blocking the entrance.

Police asked ABC News not to photograph the hospital entrance doors because it was a "sensitive situation".

A grey Mercedes Benz station wagon, which was torched and then abandoned in Chandler Lane at Rockdale, is understood to be the getaway vehicle.

The riot squad gather at St George Hospital where Mr Hawi is on life support. ( ABC News: Nicole Chettle )

Focus on preventing any acts of retribution

Police said a male dressed in all black clothing and a balaclava was seen to have committed the shooting.

"I'd say it was a targeted and planned attack and completely without regard to human life," Assistant Commissioner Mal Lanyon said.

"This is a serious, brutal and callous attack and one that we will not stand for."

A bullet hole in the passenger window of Mick Hawi's luxury car. ( AAP: David Moir )

Assistant Commissioner Lanyon said the full resources of the NSW police have been deployed to investigate the matter.

"It will be a difficult investigation, I have no doubt," he said.

It its too early to determine a motive for the shooting, he said, but Mr Hawi's history in organised crime will be considered.

Assistant Commissioner Lanyon also said there will be a major focus on preventing any retribution as a result of Mr Hawi's killing.

"It's essential when we think there could be links to organised crime that we take action to prevent that," he said.

Witnesses tried to resuscitate Hawi

People console each other at the scene of the shooting. ( AAP: David Moir )

Reshan Muthukrishna, who owns the physiotherapy clinic, said the gym was a hub for bikie gang members.

"There have been minor incidents with gangs and gang members coming to train here," he said.

"Previously there has been some instances in the car park.

"But life goes, people train, people come and go."

Reshan Muthukrishna said there had previously been minor instances with bikie members outside the gym. ( ABC News )

Mr Muthukrishna said this shooting would affect both the gym and physio clinic's clientele.

"It's definitely not good for business."

Rup Sarkar, who works at the physiotherapy clinic next to the gym, said Mr Hawi was a regular.

"Everyone knows him, he's a nice guy," he said.

Mr Sarkar said it was hard to look on as he came out of the gym to see people trying to resuscitate Mr Hawi.

"There was blood … I tried not to look."

"Not your average Thursday I guess."

Onlookers ran to help Mr Hawi in the gym car park. ( Youtube: 2GB )

One witness, who said she heard about six gunshots, was in disbelief that there was a shooting in her suburb.

"I didn't think that they were gunshots … I mean Rockdale … I don't think that that happens here," she said.

"It was kind of chilling to think it happened across from where I work or where I sometimes park my car … I won't be doing that anymore."

Another witness said she saw people coming out of the gym after the shooting who appeared "really traumatised".

Officers have begun setting up a crime scene at the Fitness First gym on West Botany Street. ( ABC News: Nicole Chettle )

Crime chief kept low profile

Sources say Mr Hawi has kept a low profile in recent years after he served a prison sentence for manslaughter over the death of Hells Angel bikie associate Anthony Zervas in a brawl at Sydney Airport in 2009.

During the airport brawl Zervas was bashed with a metal bollard and stabbed several times.

Hawi was initially convicted of murder over the incident and sentenced to 28 years in prison after a nine-month trial in 2015, but the NSW Court Of Criminal Appeal overturned his conviction on several grounds.

Mr Hawi was in his black luxury four-wheel drive when he was shot at least half a dozen times. ( ABC News )

At the time, Chief Justice Tom Bathurst, one of three judges ruling on the appeal, said he could not be satisfied beyond reasonable doubt that Hawi "inflicted fatal blows to the head of Mr Zervas using a bollard" and that "it was not open for Mr Hawi to be convicted on that basis".

The three-judge appeal panel was not unanimous in its decision.

Chief Justice Bathurst noted that his preference was to allow the appeal and acquit Hawi but indicated that Justice Lucy McCallum wanted the appeal dismissed.

The third judge on the panel, Justice Derek Price, preferred the option of a retrial.

He was released on parole in May 2015.