A decorated policeman who played a key role in putting RSA triple-murderer William Bell behind bars was let off conviction after pleading guilty to drink-driving.

After two pints at a bar and a wine with dinner, following beers at home that afternoon, Sergeant Jason Lamont got behind the wheel of his car.

The Auckland City officer - who has earned two district commendations in his 12-year career, one for the "body in a suitcase" Waitemata harbour murder in Easter 2006 - blew 512mcg when pulled over by colleagues on the North Shore last August.

He did not reveal his occupation as he "did not want there to be any suggestion of impropriety or favourable treatment", he said in an affidavit.

A fortnight ago the Weekend Herald revealed Mr Lamont was discharged without conviction for the crime - despite pleading guilty to driving at 113mg of alcohol per 100ml of blood, nearly 1 times the legal limit.

Court documents show he assured his partner Julie he was fine to drive after a "quiet evening" with family and remained confident on reaching the checkpoint.

"I was shocked when the evidential breath test gave a result of 512mcg," he said in his affidavit.

At sentencing at the North Shore District Court in April, Judge Phil Gittos said Mr Lamont should not lose his job over an incident where "he apparently considered he would not be offending against the law".

"Insofar as the relative gravity of this offending I think it is not out of the way for the court to show some leniency and to discharge the man without conviction under the provisions of Section 106 of the Sentencing Act."

Under that section a judge may rule that the consequence of conviction outweighs the crime. In an affidavit provided to the court a police source said she knew of no case where a police officer was allowed to remain working after a conviction.

Mr Lamont said his work was a top priority in his life.

"I will be absolutely devastated if I'm to lose my job over my error of judgment in the present case," he said in his affidavit.

He declined to elaborate when contacted yesterday by the Weekend Herald.

Beginning in 1998 as a general duties constable in Onehunga, Mr Lamont received several "outstanding" appraisal ratings throughout his career and frequently acted as supervisor, once at the rank of Detective Senior Sergeant.

For his work on the 2006 investigation into the partly-decapitated body of Wan Biao, found in a suitcase in Waitemata Harbour, he received a special district commendation.

His police work on the suspicious death of a baby, and its parent's subsequent conviction, earned him another.

Mr Lamont interviewed convicted murderer William Bell and helped locate Darnell Tupe, for the fatal bashing of three people at the Mt Wellington-Panmure RSA in 2001.

Judge Gittos said the serving Auckland City sergeant had "valuable and lengthy experience in the police force" and rightly feared conviction would affect the police disciplinary process.

The loss of employment for a senior police officer had "more serious consequence" than for "somebody of less prominence", he said.

"There is no reason why the taller the poppy the more likely it should be to cut it down and I think there is a valuable resource for the police to consider in their dealings in the matter later."

But Mr Lamont's experience should have spurred him to "be particularly careful I suppose", he said.

The sergeant is not the first policeman to fall foul of the law over alcohol - five officers were charged with drink-driving in the past year.

Senior Constable Ross Kneebone, 53, of Northland, pleaded guilty to drink-driving but had his Section 106 application refused.

Senior Constable Craig Fraser is before the courts next month for allegedly backing into a parked car while under the influence.

* THE LAW - A judge may discharge a person guilty of an offence where the consequence of conviction is deemed to outweigh the crime (Section 106, Sentencing Act).

* THE JUDGE - "The loss of somebody who is so far advanced and achieved such seniority in his position as this man has, is a more serious consequence [than conviction]." (Judge Phil Gittos, North Shore District Court)

* THE OFFICER - "I will be absolutely devastated if I lose my job over my error of judgment in the present case." (Sergeant Jason Lamont)