Hit & Run asserted Mapp had described the raids as a "fiasco", although the book did not quote him, and after its release he confirmed he had used those words.
In an essay published on Pundit today, Mapp said his role as Defence Minister meant he also knew people had been killed during Operation Burnham, and these were people acting as insurgents.
However, after he had retired from politics he saw a documentary by Stephenson broadcast by Maori TV in 2014, and said it became clear it was possible there were other casualties from the raids, including a 3-year-old girl, named in Hit & Run as Fatima.
Mapp confirmed he had previously been interviewed by Stephenson and he "told me enough about what had happened for it to be believable even if it was not fully proven".
"The law of armed conflict accepts that civilian casualties might occur in military operations, and in many cases there is no legal liability for them, particularly if they were accidental," Mapp wrote on Pundit.
"But for New Zealand, is that the end of the matter? Do we hold ourselves to a higher standard?
"For me, it is not enough to say there might have been civilian casualties. As a nation we owe it to ourselves to find out, to the extent reasonably possible, if civilian casualties did occur, and if they did, to properly acknowledge that."
Stephenson and Hager, Labour, the Green Party, New Zealand First and United Future have all called for an inquiry into the allegations in Hit & Run, as have lawyers acting for Afghani villagers.
Mapp said getting to the bottom of the allegations did not necessarily require an independent investigation.
Rather, information was most likely to be forthcoming through diplomatic approaches to the Afghan government and non-government organisations on the ground in Afghanistan.
Despite the NZDF maintaining Hit & Run contained major inaccuracies including the location of the raids, and Hager and Stephenson's subsequent admission the location given for the villages in the book was incorrect, Mapp said the accounts of the NZDF and the two journalists were reconcilable.
That was because NZDF had now recognised that civilian casualties may have occurred.
Mapp concluded by saying New Zealanders have good reason to be proud of the professionalism of its defence forces.
"The SAS are among the most highly trained and respected soldiers in the world. In our name, we ask them to undertake the most hazardous military missions, often deep within enemy-held territory.
"They have an absolute right to defend themselves against attack. The risk of capture of our soldiers by the Taliban would be beyond contemplation.
"Part of protecting their reputation is also finding out what happened, particularly if there is an allegation that civilian casualties may have been accidentally caused.
"In that way we honour the soldiers, and also demonstrate to the Afghanis that we hold ourselves to the highest ideals of respect of life, even in circumstances of military conflict."
New Zealand First leader Winston Peters said the NZDF's integrity had been seriously challenged by the Hit & Run allegations and he agreed with Mapp to the extent further investigation was needed to provide certainty about what happened.
"We need to clear the decks here," Peters said. "There has been a less than compelling response from NZDF."
Peters said he was concerned with the facts and not judging Mapp's decision to talk to Stephenson, but said the former Defence Minister was motivated by a "desire to tell the truth as he sees it".
