Privacy Commissioner Marie Shroff has formally launched an investigation into the blunder which saw ACC send private information about more than 6000 clients to a disgruntled claimant last year.
Ms Shroff said the investigation would also cover ACC's standards for securing personal information.
However, she also said it appeared the breach only involved ACC claims that were under review in August last year.
"If your claim was not under review at that time, it is likely that you are not affected.''
It appeared the information did not contain medical or "accident'' details, she said.