Meanwhile, it appeared the case manager, who entered details of accidents and injuries, claim numbers, bank account numbers and contact details for claimants as an "aide memoire" when working outside of her home base of Christchurch, breached ACC privacy rules by taking the notebook home.
The corporation had only had "a limited opportunity" to discuss the matter with the case manager who was on leave due to a family illness, "but it appears the policy was not adhered to on this occasion", Mr Miller said.
ACC came under sustained fire last year for a series of privacy breaches including details of thousands of claims mistakenly emailed to disgruntled claimant Bronwyn Pullar.
Those breaches prompted an independent review of the corporation's information handling procedures which found "systemic weaknesses within ACC's culture, systems and processes" and recommended a series of improvements.
Mr Miller said ACC had done "a large amount of work" relating to those recommendations and this month's incident was "particularly disappointing" in that context.
Nevertheless, he defended the corporation's performance, saying "great improvements" had been made, "and in terms of the number of breaches, the trend is certainly coming down'.
Prime Minister John Key, who has downplayed the significance of privacy breaches at ACC the Earthquake Commission and other state sector agencies, said ACC dealt with a a great many claims".
"The number of media reports of privacy breaches has reduced significantly in recent times so I actually think they're doing pretty well."
ACC incidents
* October 2011: ACC sends private information of 6500 claimants to former National Party insider Bronwyn Pullar.
* May 2012: Two ACC claimants lodge formal complaints after their confidential files go missing while their doctor is travelling on a bus.
* July 2012: ACC sends an Auckland builder a five-page document about a Te Atatu brain injury victim in the middle of a bundle of documents about his own case.
* October 2012: ACC sends a Waikato man details of another man's criminal history.