Kiwis get to vote on the preferred alternative between November 20 and December 11.
Mr Key texted McCaw, probably the most famous living bearer of the emblem, sometime between the August 13 release of a video in which the Prime Minister set out his reasons for supporting change.
The Prime Minister's chief of staff Wayne Eagleson said there were a number of text messages between Mr Key and Mr McCaw and it was unclear whether they would be classed as "official information".
"Most were of a general nature (and not necessarily as Prime Minister) such as text messages from the Prime Minister stating 'good game'."
But he said one specific text was directly relevant to the flag debate. "The Prime Minister's recollection was that he sent a text message to Richie McCaw that suggested that he look at the Prime Minister's Facebook post on the flag.
Mr Eagleson said McCaw responded "he would have a look".
Towards the end of August, McCaw backed Mr Key's call for a silver fern flag.
The exchange adds McCaw to Mr Key's list of text buddies, which has previously included Whaleoil blogger Cameron Slater and gossip columnist Rachel Glucina.
The video was immensely popular on Facebook - it had 1,026,927 views by early this afternoon. In it, Mr Key said the change would be important and benefit New Zealanders.
He said "sheer confusion" was a good reason to change the flag as it was often mistaken for the Australian flag.
A strong reason for changing the flag was to recognise the deaths abroad by servicemen and women with silver ferns marked on their headstones.
"They really fought for the values and principles that underpinned this country - for democracy, for freedom, basically for human rights, for women's rights. These are the things our people fought for, sought to defend."
The most important reason for change was to "build over signs of patriotism" and to "celebrate this young country".
He said he backed the silver fern and a flag people were happy to wear on their clothing.
The response to Ms Coleman's Official Information Act request also revealed the designer of two of the four-strong shortlist flags had lobbied Mr Key in recent years. It revealed Kyle Lockwood sent Mr Key versions of the flag which appeared in the shortlist, and fern lapel pins.
The OIA response also showed opposition from three regional branches of the Returned Services Association, and from the national body.
Ms Coleman said she believed the approach to the flag change project was wrong and she wanted to know more about it.
"I felt the whole flag consideration process was flawed from the beginning. I couldn't believe it when three of the final four flags announced included a fern and two were by the same designer."
She sent OIA requests after coming to feel the process was "a bit stage-managed and superficial".
She said the extension of time needed to deal with her request appeared to validate recent concerns the OIA process was being used to delay public access to information.
Text messages were confirmed as "official information" following an inquiry by the Chief Archivist after Mr Key said he regularly emptied his phone for space and security reasons.
The Chief Archivist found some deleted messages should have been kept and briefed Mr Key's office on what should be kept.
