"The nature of the relationship seemed to be, to some people, rather intense and that was creating complications for a number of the officials."
It was understood that Ms McGregor had made a complaint of sexual harrassment to the Human Rights Commission against Mr Craig.
The board had previously confronted Mr Craig about the matter, which Mr Stringer described as a "dangling sword hanging over us for quite some time".
Colin Craig had earlier denied any board members had raised concerns about his relationship with Rachel McGregor or accused him of being dishonest.
"That is completely untrue, because the board has discussed this almost monthly for nearly a year," Mr Stringer said.
The board had called a special meeting yesterday to discuss Mr Craig's leadership and his relationship with Ms McGregor, but the meeting postponed by Mr Craig for his press conference.
Mr Stringer said he had no right to do so.
He said the board had also addressed Mr Craig's infamous sauna interview on TV3's late night show Newsworthy, which they questioned his judgement over.
Chaperones had been appointed to Mr Craig to try and shift public perceptions of him, Mr Stringer said.
In light of the current accusations against Mr Craig, it was "completely untenable" for him to continue as leader, he said.
In a statement this afternoon, chairman of the Conservative Party, Brian Dobbs, said Mr Stringer's appearance and comments on TV3's The Nation this morning was in his own capacity, and was not in any way as a spokesperson for the Conservative Party.
"The board has a number of important matters to discuss, however his comments were his own opinion not representative of the board."
Mr Dobbs said as advised yesterday, the board had agreed to the Party leader stepping down, and would now engage in a selection process that would involve consultation with regional committees, party members, and supporters.
He said the board would make further comment as it progressed with these processes.
