A South Auckland police officer says she's been criticised and told to resign after getting pregnant.

An anonymous letter by the Police Sergeant was published in the Police Association's Monthly magazine, outlining the treatment she's received from the service.

She says when she told her supervisor the news, he said he knew he shouldn't have hired her.

She claims she was told to resign multiple times, and threatened that if she didn't, she might be moved to anywhere else in the district on her return to work.

She's stressed about returning despite loving her job for 10 years, and says if wasn't for her Auckland mortgage, she would not go back after having a baby.