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One of the teenagers, a 15-year-old boy, was later taken by ambulance to Dunedin Hospital with alcohol poisoning.
A 75-year-old man has been charged with supplying alcohol to a minor.
The boy and his friend, a 16-year-old girl, arranged for the girl's grandfather to buy a bottle of vodka on Saturday, May 17, alcohol harm prevention officer Sergeant Ian Paulin, of Dunedin, said.
The pensioner drove the pair to a South Dunedin liquor store, where he bought a bottle of vodka with cash from the boy and ''topped up the amount for the granddaughter''.
The man then dropped off the pair to a party in the student quarter, Sgt Paulin said.
''It should have been a massive red flag.''
After sharing the vodka with the girl, the boy became ''extremely intoxicated'' and was taken to hospital, where he was treated for alcohol poisoning.
The boy's parents laid a complaint with Dunedin police and the girl's grandfather was spoken to about the incident earlier this month.
He will appear in the Dunedin District Court next week.
Sgt Paulin said it was the first time Dunedin police had charged anyone under that section of the Sale and Supply of Alcohol Act (2012).
''The only person who can supply alcohol to a person under 18 is their parent or legal guardian, or someone with the expressed consent of their parent or legal guardian ... and that wasn't the case here.''
Sgt Paulin confirmed the grandfather, who was not the girl's legal guardian, did not have consent from the parents of either minor.
The man faces a maximum fine of $2000.
Earlier this year, the Otago Daily Times reported police concern about the number of high school pupils gatecrashing student parties, after two grossly intoxicated teens were taken to a police cell for detoxification.
''It has been highlighted in the media about the inappropriate consumption of alcohol in the student area and then to put a 15-year-old and a 16-year-old in that sort of risk ... it is beyond comprehension,'' Sgt Paulin said.
Last week, two adults were fined for supplying alcohol to a 9-year-old boy filmed in a drunken state in a Hamilton skatepark earlier this year.