A Nelson mother with a vendetta against a vulnerable mentally impaired man has been jailed for setting him up by falsely telling the police the man had sexually assaulted her daughter.

Tracee Marie Deane, a 39-year-old mother, was jailed yesterday for two years and eight months for her part in the hoax, which Judge Tony Zohrab said was an attack on "the heart of the justice system".

Four of the five children Deane is mother and stepmother to were in court for the sentencing and sobbed as their mother was led away.

Judge Zohrab said Deane's offending was serious and it must have been very frightening for her impaired victim who was arrested, held in the police cells and released on strict jail conditions as a result of the false claims.

Deane earlier admitted one charge of conspiring to bring a false accusation and other charges, including two charges of using a document to obtain a pecuniary advantage and two charges of theft.

Judge Zohrab said on May 7 last year that Deane and her daughter were at the Nelson Public Library using the computers with a friend. The victim, who lived in community flats, was also using the library's computers, but left the library about 10.15am. A history of animosity existed between Deane and the victim.

Deane, her daughter and friend also left the library and on a drive to Stoke discussed how they could set up the victim.

The trio returned to the library and, after a short while, Deane's daughter phoned the police and told them the victim had grabbed both her breasts from behind and had also groped her through her clothes.

Deane told the police she heard her daughter screaming and saw the victim pulling his hands off her daughter. The Deanes' friend also made a statement supporting the allegations.

As a result of the three statements, the victim was arrested and charged with indecent assault, which he denied.

However, after watching the library's security-camera footage, the police became concerned about the allegations.

The police reinterviewed the Deanes' friend, who was also mentally impaired and told officers Deane had told him to lie about the incident because she hated the victim.

Judge Zohrab said Deane's daughter also admitted lying about the assault at the encouragement of her mother and the charges of indecent assault were dropped. The victim was angry at being set up and not being believed by the police, he said.

Deane's daughter, who Judge Zohrab said lied to the police about the attack under Deane's encouragement, was last year sentenced to five months' home detention for her part in the hoax.

Deane's lawyer, Wayne Jones, said Deane had only offended in the past two years, which she blamed on mixing with the wrong people, but she no longer associated with them.

Judge Zohrab said he was unable to give Deane a jail sentence of less than two years, which would allow her to qualify for home detention, because of the serious nature of her offending.

Deane had also committed other dishonesty offences against vulnerable victims, including stealing money from a woman in a community home.

Deane's husband, Darren Deane, said yesterday his wife had been "slammed with a jail sentence" out of proportion to her offending and her jail sentence would be incredibly hard on the family. The youngest child was 11.

"My wife is not a threat to society. She was only a menace."

He believed his wife had been set up to make the false accusation.

He said the only reason his wife did not like the victim was because her daughter "got tangled up with him".