They reportedly went under water and failed to surface, police said.
An air, water and ground search has been under way for the past two days.
Police have extended their condolences to the girl's family and loved ones.
On Tuesday, a body believed to be that of a missing swimmer was recovered from the Waimakariri River in Canterbury.
The 18-year-old had been missing since December 22 and was one of many to die in a spate of water-related incidents across the country the past week.
Six people have died since Christmas Eve while a 2-year-old Raglan boy is valiantly fighting for his life after an incident in a pool on Boxing Day.
