"ACC does not know why dissatisfied people do not complain, and has not carried out any recent research to find out why," Ms Provost said.
However she noted that "if people see little value in complaining, then the number of complaints might fall because people's confidence in the system and processes is weakened".
Ms Provost made five recommendations to improve ACC's complaints handling process, including seeking and implementing measures to make it easier for people to complain, maintaining a consistent record of complaints and the corporation's response, and better reporting of its performance in handling complaints.
ACC chief executive Scott Pickering said it would implement all five recommendations.
The Office of the Auditor General (OAG) also released a report on the Ministry of Social Development's complaints process and found while it generally dealt with complaints professionally and had changed some of its practices as a result of complaints, it could take too long to deal with them and complainants were not kept well enough informed.
The OAG contracted a research agency to survey 669 people who had complained about Work and Income and StudyLink, and conducted fuller interviews with 10. They found while most believed their issue was resolved fairly in the end, only 36 per cent were happy with the way the complaint was handled.