A woman once described as being "highly motivated to seek out children, particularly pre-pubescent boys" has been caught loitering in Christchurch yet again.

Margaret Dodds, 60, was spotted outside Thorrington School in Cashmere on Thursday afternoon.

Principal Christine Harris alerted parents in an email to watch out for Dodds. She told some pupils to take an alternate route home.

John Kirk-Anderson Margaret Dodds leaving court in February 2014.

"It is important that you remind your children again about not talking to strangers as Margaret will try to engage them in conversation."

Police said the school alerted them about 3.20pm. By the time they arrived Dodds had left.

Dodds has a long history of following and lingering around Christchurch children, which has been described by one judge as "very frightening".

Dodds has convictions for assault and trespass. She has been trespassed from buses, malls and many public facilities.

Harris told parents that, while police could move Dodds away from the school, they could not legally move her on from nearby streets.

"Unfortunately in ensuring that children were safe I had to explain to some of them that they had to take alternate routes home or walk with me to the office.

"This may have been very upsetting for some children so please take the time to discuss this with them."

Dodds is a long term resident of the Seager Clinic in Princess Margaret Hospital, a mental health facility.

In 2010 Dodds served what was then a third prison term for her 19th attack on a caregiver. The court was told of incidents in which she threw chairs and a rubbish bin at staff members.

In 2014 Dodds was jailed for six months after pinning a 10-year-old girl against a tree with her stomach and accosting three boys in a separate incident.

Dodds was jailed again for four months in 2016 for breaching her intensive supervision order.

Within hours of returning to her clinic after that sentence, Dodds left the clinic and was found at a bus stop staring down a young family, which included a boy aged under 5.

After resisting probation officers she was arrested again and sent back to jail for another four and a half months.