A woman with a history of intimidating children has been arrested amid allegations of her watching children in a Christchurch park.

Margaret Dodds was spotted by a concerned parent at a Cashmere park watching children, according to a social media post on Sunday that warned others to be vigilant.

Police arrested a 63-year-old woman for trespass on Cashmere Rd on Sunday, a spokeswoman said. The woman, understood to be Dodds, is due to appear in court on Tuesday.

JOHN KIRK-ANDERSON/STUFF Margaret Dodds has a history of harassing children. (File photo)

Dodds has a long history of following and lingering around children – behaviour described by one judge as "very frightening". She has been described as highly motivated to seek out children, particularly pre-pubescent boys.

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Dodds has been a long-term resident at the Seager Clinic, an inpatient mental health service at Princess Margaret Hospital. The Canterbury District Health Board said it could not comment for privacy reasons.

Her lawyer at a previous appearance said her condition made it "mission impossible" for her to stay out of trouble. Dodds has assault and trespass convictions, and has been trespassed from buses, malls and many public facilities. An intensive supervision sentence, which included a condition that she not have any contact with children under 16, finished in mid-2018.

Warning messages have been posted around school staffroom noticeboards and her outings around the city often lead to messages and photographs being posted on social media.