No-one was injured in the incident.
Detective Senior Sergeant Dave Pizzini from the Manurewa CIB said police are concerned about the use of a sawn-off shotgun, described as around 40cm long.
"It is concerning to us that a firearm was used in this crime and particularly that it was discharged in the store with members of the public present,'' he said.
"We urge anyone who witnessed the car being stolen or people running from Corin Avenue to come forward to assist the investigation.''
At least a dozen witnesses have been interviewed and CCTV footage from the store is expected to provide more information about the offenders, Mr Pizzini said.
The gunman has been described as Maori or Polynesian, about 170cm to 180cm tall. He was wearing a dark hoodie, black jeans, a dark beanie hat and had his face covered by a dark bandanna with white marks.
No description of the second robber is available.
Anyone with information is asked to contact Manurewa CIB on 09 295 0200 or anonymously through Crimestoppers on 0800 555 111.