Maurice Williamson's resignation as a minister was forced by "a significant error of judgment" in the words of Prime Minister John Key.

As soon as Key discovered that Williamson had called the police about their investigation into domestic violence allegations against businessman Donghua Liu, Key had no option.

It was clearly not a close call, but a clear breach of acceptable behavior.

The fact that Williamson said he was not trying to interfere in the police process - and even made that clear to the police - is no mitigation.

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The call was made and that, of itself, constitutes interference, whether intended or not.

Emails obtained by the Herald's Jared Savage about Williamson's call to the cops state:

"He [Williamson] started by saying that in no way was he looking to interfere with the process - he just wanted to make sure somebody had reviewed the matter to ensure we were on solid ground as Mr Liu is investing a lot of money in New Zealand."

The puzzle is why Williamson made such an error when former colleagues have resigned for much less.

Former ACC Minister Nick Smith resigned in 2012 after the Herald revealed he had written a personal testimonial on his office letter head in support of ACC claimant and friend Bronwyn Pullar.

And in 1999 deputy Speaker and former Northcote MP Ian Revell was forced to resign for trying to get off a $40 parking ticket using his parliamentary letterhead, then telling the North Shore district commander he would not be supporting his reappointment.

Key easiest decision was in deciding that Williamson crossed the line.

The hard part will be addressing the perception that his ministers will breach accepted standards of proper conduct to help wealthy friends and party donors.

Read more of the Herald's coverage of this story today:
Maurice Williamson resigns as a minister
Audrey Young: Why Maurice Williamson should be sacked
Emails between top cops revealed
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