Mt Pleasant School, where threats have been made against children.

A Christchurch school community is in shock after two families received letters threatening to harm their children and demanding money.

Police said the families, who live in the hill suburb of Mt Pleasant, found the letters in their mailboxes over the past week.

Both families' children attend Mt Pleasant School, where staff are on alert.

A Mt Pleasant School pupil's mother said she was shocked threats of this kind had been made in a safe neighbourhood.

"I hope it's someone's idea of a sick joke," said the woman, who asked not to be named.

"We chose to live in Mt Pleasant because it has such a low crime rate so it is very strange it's happening in this neighbourhood."

She did not know who the families who had received the threats but she felt "really, really worried for them".

"As a fellow parent I can really empathise with them."

She was pleased with the school's handling of the situation and felt her daughter was safe and well taken care of during school hours.

Detective Inspector Greg Murton said police took the matter "very seriously". A team of detectives were trying to identify the letters' author.

"We have also put in place additional security for the families involved and have increased police patrols in the area.

"While there is no evidence of a genuine threat towards any children at Mt Pleasant School, parents have been advised of the existence of the letters."

It was too early in the investigation to draw any conclusions. Police would not speculate about who had sent the letters.

Police were confident that the steps taken would ensure the safety of the children and families involved.

"Mount Pleasant School has contacted all of the parents who have children at the school to advise them of the current situation," Murton said.

Mt Pleasant School board of trustees chairman Marty Anderson said one of the two letters referred to Mt Pleasant School students.

The school had immediately put additional safety measures in place, he said.

Mt Pleasant School principal Chris Nord said staff had talked to parents to ensure they were aware of the situation.

Sumner School sent out a "parent alert" this morning, asking parents to be "consider how your children are currently getting to and from school and to think carefully before allowing your children to play anywhere on the school grounds without adult supervision".

The school said the threats were "obviously of great concern for all the families in the Mount Pleasant community".

"The advice we have been given is that in all likelihood this is a localised threat and there is nothing to suggest that our children are in any danger."

The school had informed staff about the situation and asked them to be particularly vigilant around bell times and while on duty.

Anyone receiving similar threats should contact police on 363 7400.