The youth has since been charged with manslaughter.
Counties Manukau South Area Commander Inspector Dave Glossop confirmed police were investigating the death.
He said the injured security guard was taken to Middlemore Hospital but died early this morning from his injuries.
"Police have arrested a 17-year-old male in connection with the assault and inquiries are ongoing.
"We cannot rule out the possibility of charges being laid."
Glossop said he could not comment further as the matter was before the courts.
The victim's name has not been released.
It is understood police are still notifying his family overseas.
The guard had worked at Countdown for eight months.
"We are deeply saddened by this tragic loss of one of our team," Countdown managing director Dave Chambers said.
"The safety of our people and customers is our absolute priority, and no New Zealander should go to work and not expect to return home to their loved ones.
"Our focus right now is to support the family and our team in whatever way we can.
"We've also made as much information and footage as we have available to the police and will continue to cooperate with their investigation."