The Independent Police Complaints Council (IPCC) says it has lost some documents containing personal information on 20 people, including complainants and police officers.
In a statement, the council said a folder, containing six complaint reports, was left in a taxi on Monday evening. It did not specify who had lost it.
The reports detail the names, identification numbers, ages and ranks of 13 police officers who are the subject of complaints.
They also include the names, ages and occupations of six complainants, with partially concealed identity card numbers, addresses and phone numbers. The name and occupation of a witness is also included.
The IPCC said it had reported the loss to the police and the privacy commissioner, adding that it would contact the 20 people affected.
It appealed to anyone who finds the folder to hand it to the police or to contact the IPCC.
The council said it has put in place an email system for the encrypted transfer of restricted documents in light of the incident.