Northland police are warning the public about people posing as officers in the area.
The most recent incident was on Sunday when a red Holden Commodore with flashing red and blue lights stopped another vehicle on Otaika Road in Whangarei.
One of the four men in the car showed an ID and searched the car boot for alcohol, before asking the female driver for her identification and how much money she had with her.
When the woman questioned the validity of the stop, the man said they had another job to go to and the car sped away.
It was the second such incident in February.
Northland area commander Tracy Phillips told Radio New Zealand's Checkpoint programme on Wednesday she is concerned that the crimes could escalate, as the men are getting bolder and braver.
"We take it very seriously, because this impacts on public trust and confidence in police.
"We know that these guys have stepped up what they're doing and I shudder to think that if they hadn't been challenged by these women, in the most recent complaint, what they might have done."
Ms Phillips says if officers are not in uniform when they stop vehicles, they will produce an ID with a name, photograph and police identification number if asked.