The defendant, in his 50s, admitted three charges of indecently assaulting the 10-year-old but in an interview with a probation officer he denied the incident was sexually motivated and was done to provoke a reaction in the girl.
His explanation that it was ``simply to wind her up, with no element of sexual arousal on your part is implausible'', Judge Michael Turner said.
At yesterday's sentencing, the Dunedin District Court heard the family had been torn apart by the events of July 29.
The grandfather went to his son's Dunedin home to visit the family and took his granddaughter back to his house about 5.30pm.
Judge Turner said the pair, whose ages were almost 50 years apart, began playing with some exercise equipment.
When the defendant placed his hand under the victim's top, she told him to stop.
But the man just groped her in a different area.
Later the girl was practising handstands in the living room. Her grandfather pretended to help by holding her legs as she was upside down but it just exposed the girl to another indecent assault.
``You offered her money not to tell anyone what you'd done,'' the judge said.
When the girl got home she told her parents about the molestation, which resulted in her father confronting his father.
He punched the defendant when he did not deny the allegations.
The incident had torn the family apart, the court heard.
The defendant tried to kill himself by overdosing on prescription medication and his son was later admitted to hospital because of overwhelming stress
caused by the situation.
Defence counsel Anne Stevens said her client's wife had stood by the man, which had meant she had not seen her granddaughter for more than four months.
The couple had previously moved house to be nearer the victim's family and she had had to shift again while her husband was in custody awaiting sentencing.
Mrs Stevens said the defendant should be given a term of home detention and required help.
``He's scared of confronting what he's done,'' she said.
``He needs professional people who are trained to get through those difficult periods, to get through to the core of why this could have occurred. The sooner that professional support can be provided, the greater the benefit for the community at large.''
Probation noted some concern that the prospective home detention address was about 300m from a primary school but Mrs Stevens said it could not be seen from the house.
Judge Turner agreed with Crown prosecutor Robin Bates that the man's offending was too serious for any sentence short of imprisonment.
``I have significant reservations about whether you fully accept your conduct, the reasons for it and the consequences from it,'' the judge said.
The court heard how the victim felt ``sad'' and annoyed about what she had been put through.
``She says she'll never be able to trust you again.''
The grandfather was jailed for one year and five months and is subject to six months' post-release conditions which will require him to complete counselling and bar him from contact with under-16s.
The defendant will also be placed on the Child Sex Offender Register.
