The file left on a police car dashboard detailed an indecent assault against a woman earlier this month.

A police investigation is under way after a police officer left classified documents about an indecent assault on the dash of their car outside a supermarket on Monday.

A passerby emailed a photo to Stuff of case files left in the vehicle outside the Countdown supermarket on Quay St in downtown Auckland.

The file, which was said to be "clearly visible on the dash", detailed an indecent assault against a female, including her name and address.

The person who sent the photo to Stuff said it was "disgraceful".

Auckland City Police Superintendent Karyn Malthus said police were making immediate inquiries into "this apparent breach of privacy where a file has been left visible in a police car".

"Police are taking this matter very seriously and inquiries are underway to establish exactly what has happened," she said on Wednesday evening.

"We have spoken with the victim this evening to apologise and will ensure that she is supported," Malthus said.

Malthus said this type of error was unacceptable.

She wanted to assure the community that the privacy of victims was paramount to police.

"We deal with tens of thousands of assault matters each year and have rules and procedures in place to protect the privacy of these victims."

Malthus said police were unable to comment further, "except to reiterate that the privacy of victims is paramount to police."