Police Commissioner Howard Broad said there were exceptional circumstances, which will probably never be made public, in the case of a top cop who refused a breath test after being suspected of drink driving.

Superintendent Graham Thomas, head of prosecutions at police national headquarters, was cleared by an internal investigation and is now on paid sick leave.

Witnesses reported seeing Mr Thomas driving erratically but when police officers knocked on his front door and asked him to take a breath test he refused.

He was legally allowed to refuse but as a senior member of police he would be expected to voluntarily take the breath test, Mr Broad said.

He said the matter then became an employment one.

Mr Broad was questioned about the incident when he appeared before Parliament's law and order select committee for a routine financial review today.

"The legal matter ends with the ability to prove the officer was above the legal limit to drive."

The internal employment inquiry "received an answer" from Mr Thomas and accepted the answer but cannot make it public for privacy reasons, Mr Broad said.

He stressed any citizen would be treated the same as Mr Thomas and there was no double standard.

"We would take, and I do take, the issue of a senior officer who drinks and drives and then fails to apply with our code of conduct very seriously."

But, there were "unique circumstances" in this case which made full disclosure difficult.

"The senior officer concerned is currently on medical rehabilitation that is continuing ... There are residual issues that we need to deal with and I will," Mr Broad told reporters after the select committee.

"I'm concerned for the welfare of my staff and I place that responsibility very highly.

"In normal circumstances, I would always place the maintenance of the confidence of police above the interest of individual staff members."

Had the issue occurred under normal circumstances it is likely the details would have been made public, Mr Broad said.

Yesterday, Police Minister Judith Collins told reporters it was an employment matter but it was "not a good look, I don't think it's very helpful at all for the police".