The police detective acquitted of exposing himself to a woman and her 11-year-old son will now face an internal police inquiry to find out if he keeps his job.

Thomas Andrews, who works at police national headquarters, was charged with obscene exposure and offensive behaviour after a drinking session on New Year's Eve.

Judge Sharon McAuslan dismissed both charges at a defended hearing in the Pukekohe District Court yesterday.

Police spokesman Jon Neilson said Mr Andrews, 44, will now face an internal police disciplinary process, which is standard practice after an officer faces criminal charges.

Despite throwing the charges out, Judge McAuslan criticised Mr Andrews for being highly intoxicated in public and trying to carry out police work in such a state.

"I find it reprehensible that a serving police officer was so intoxicated in a public place," she said.

As part of his evidence, Mr Andrews said he had had about 10 drinks while out dancing in Pukekohe before heading to a BP station for food about 2am.

He said he stopped to urinate in central Pukekohe and shortly after, he heard a "metallic sound" and saw lights outside the Salvation Army shop, so decided to investigate.

Justine Richardson said she was confronted by a drunk man who swore at her.

"The words were slurred and not flowing and that's when I became aware he was intoxicated," Ms Richardson told the court on Wednesday.

She said the man showed her a detective's badge before she told him his fly was undone.

"He then looked down and he chuckled a little bit and stepped backwards.

"He began to fumble with the dome on his pants and he was still chuckling as if he found the situation very funny. It was then that his pants fell down to his knees area. I saw his penis," she said.

But Mr Andrews told the court this never happened. He said he confronted a woman while only moderately intoxicated and she sped off. He told the court that at no time did he expose himself to Ms Richardson.

Judge McAuslan said she had doubts about Ms Richardson's story and pointed to evidence raised by defence lawyer Peter Kaye that Ms Richardson had a history of dishonesty offences.

Ms Richardson had earlier told the court that after the exposure she had driven around the block before going to the 24-hour BP petrol station to call police.

Judge McAuslan said she found this strange, given there was evidence both she and her son had cellphones, yet neither had called police.

She also found it strange that the woman's son had turned away from the man at the window just as the exposure was supposed to have happened.

"If a child is so scared he would have remained [focused] on the subject of terror."

Ms Richardson said that it was difficult to bring a complaint against a police officer and the experience has had a negative effect on her son.