It takes a fair degree of provocation from someone or something for Key's temperament to shift from Mr Amenable to Mr Angry. But he was seething as he delivered a very blunt and very public dressing-down of his front-bench minister.
He also delivered a very public notice that she was now on a final warning and that he would not want to be in her shoes if there was any repeat of the serious blurring of the lines between what is and is not acceptable from a minister.
Caution still prevailed, however. Sending Collins into exile would have been a very big call - one which might have been seen as harsh given her otherwise impeccable record as a minister.
It would have risked turning an MP who is popular in the wider National Party into a potential martyr and allow her to become a magnet for any caucus and party dissent - plus the freedom to lay out a vision of the direction she would take National if, as is probable, she challenges for the party's leadership when Key eventually steps down.
Leaving her where she is has its advantages. She is a highly effective minister who has handled difficult portfolios such as Police and Corrections with aplomb.
Equally worrying for those who would push her barrow has been her refusal until yesterday to take a step back, draw a long breath and admit that, yes, she had made some mistakes which she now regretted and for which she apologised.
That would have instantly ended the fun Labour has been having. Instead, Collins allowed what she described as "a 20-minute cup of tea on the way to the airport" to turn into a 10-day-long distraction from the problems Labour has been having.