Sergeant Tony Woodbridge, of Oamaru, said a group of about five young people had been jumping off Holmes Wharf and paddling in the harbour when the 13-year-old girl was attacked.
Emergency services were called to Friendly Bay about 4.30pm, and witnesses said the shark was dark grey and about 1.8m long, Sgt Woodbridge said.
It was believed to be a sevengill shark, which were often spotted in the Oamaru Harbour area. There had been several reports of sightings on social media over the past month.
The 13-year-old girl had several puncture wounds and was taken to Oamaru Hospital for treatment.
Sgt Woodbridge said the girl was in good spirits after the attack and the injuries were not life-threatening.
"It’s good it’s not a fatality ... she will live to tell a pretty amazing tale," he said.
The attack came as a shock to the group of young people swimming in the harbour, one of them witnessing the girl being gripped in the shark’s mouth and shaken around, he said.
Sgt Woodbridge advised people heading into the water to keep a watch for sharks and be cautious.
