Then-ACC Minister Nick Smith wrote a reference for his friend and National Party insider Bronwyn Pullar as she was battling to secure entitlements from the corporation.
Dr Smith said last night it was sent "as a friend" in July last year to provide evidence of his knowledge of her state of health before her cycling accident in 2002.
He said the letter was written only so Ms Pullar could give it to her medical assessors.
It is not known if she used it while pursuing her claim with the corporation.
Ms Pullar was last year accidentally sent information about 6700 other claimants, including data which identified some as making "sensitive claims" for injuries resulting from rape or other forms of sexual assault.
ACC alleges Ms Pullar tried at a meeting with senior managers in December to secure benefit entitlements for two years as a trade-off for returning the information. Her friend, former National Party president Michelle Boag, was her support person at the meeting.
The Herald was told of the reference from Dr Smith by a woman who said she was an ACC claimant.
The informant was resentful at what appeared to be preferential treatment.
"Why did Dr Nick Smith ever write her a letter in his capacity as ACC Minister on Parliamentary letterhead?
"Us claimants who do have problems and issues over a long period of time, we don't get to use high powered people, we don't get to have meetings with senior managers or the board.
"How come she gets those kind of privileges as a National Party member?"
Asked whether Prime Minister John Key was aware Dr Smith had written such a letter and if it was appropriate, a spokesman last night said Mr Key knew of the reference but heard about it only late yesterday afternoon.
"But he was aware that on more than one occasion the Minister wrote to the person concerned saying it would be inappropriate for him as ACC Minister to get involved in her case."
Dr Smith said the letter was written last July in his capcity as Ms Pullar's friend.
"Ministers still have friends and providing there is no inappropriate influence, it is quite appropriate for them to provide information or evidence for medical assessments and other legal processes."
His "best recollection" was that the letter was written on plain paper. Dr Smith said Ms Pullar had emailed him many times seeking his intervention in her case after he became minister in 2008 but he had declined saying it would be inappropriate.
He had also advised ACC that she was a friend and that it would be inappropriate for him to have any involvement in the case.
Labour deputy leader Grant Robertson said that as Dr Smith was ACC Minister at the time he wrote the letter, he was was "hopelessly conflicted".
Earlier yesterday Mr Key confirmed he, too, knew Ms Pullar and met her when he entered politics, shortly after the bicycle accident in which she suffered head injuries.
"For a number of years she had mentioned to me, when I was at events, her frustration with ACC, so it was well known."
Mr Key did not believe it was inappropriate for Ms Boag to act as Ms Pullar's support person during the December meeting with ACC officials Philip Murch and Hans Verberne.
ACC's account of the affair has been disputed by Ms Pullar and Ms Boag. ACC has now given its files on the matter to the police.
Ms Pullar yesterday denied ACC's allegations.
"I made no threats and no demands of ACC,'' she said. "I made no threats and no demands for return of the information. I did not threaten ACC that I would inform the media of the alleged privacy issue. I did not threaten ACC to get my own way in any way. "ACC were advised the file had been sent by one of their staff. "The purpose of the meeting was to discuss how ACC could most effectively support my rehabilitation and return to work."