In that report, the Office of the Auditor-General (OAG) said ACC was not doing enough, at various levels of its organisation, to learn from complaints.
And a survey undertaken by the OAG showed that only 22% of ACC claimants who had made complaints had been satisfied with the outcome.
The OAG recently reviewed ACC matters since the earlier report and said ACC had "made some good progress", but it was yet to assess "whether its changes have improved its complaints management and complainant satisfaction".
The OAG said the ACC work needed to continue, so that it could "use complaints information to improve its systems and services".
And not enough had been done to meet another recommendation: that ACC provide senior leadership, the ACC board, and the public with "accurate, reliable and appropriately detailed information about complaints" and how the organisation had learned from them "to make service improvements", the OAG said.
Dr Powell said ACC had clearly made some improvements since the 2014 report, and any improvement "has to be a positive".
But ACC had not done enough to listen closely to grass-roots ACC claimants with complaints, to learn "where the complaints start out".
And inadequate reporting of complaints to the ACC board represented a "lost opportunity" to learn more and further improve performance, she said.
ACC should further emphasise the "human rights" approach which had been highlighted by a United Nations committee monitoring New Zealand's compliance with the Convention on the Rights of Persons with Disabilities.
ACC spokesman James Funnell said ACC took all complaints "very seriously".
The OAG's report "acknowledges the good progress we have made since 2014, and that we have more work to do".
ACC agreed, and was "progressing a range of initiatives focused on improving our customers' experiences", which would address all the auditor-general's recommendations.
These initiatives included a project called Heartbeat.
This would "allow us to receive real-time feedback from customers about their experience" - so that "immediate co-ordinated action" could be taken to address issues as they occur, and to monitor trends affecting the customer experience.
ACC had also developed a high-level design for a "navigation service" which would be capable of advocating for clients' interests, assisting them to raise complaints, and helping them to prepare for a review hearing if required.
It hoped to launch the service by the middle of next year.
ACC was also working with the Government Centre for Dispute Resolution to complete an assessment of the ACC's dispute resolution framework against best practice principles, Mr Funnell said.
