Roderick Matthews has had his original sentence quashed and substituted with a term of one year and seven months.

A man who fractured his baby daughter's leg because she was crying while he was having sex with his partner has had his prison sentence reduced on appeal.

Roderick Matthews was convicted for causing grievous bodily harm, including a fractured leg, to his 6-month-old daughter and was jailed for two years by district court Judge David Saunders.

The injuries happened when Matthews and his partner were having sex while their daughter lay on the bed next to the pair.

The girl's leg became trapped under Matthews' knee, and she began to cry. He became angry when his partner told him to stop the sexual activity and, without lifting his body weight off the baby, grabbed one of her limbs and attempted to yank her out from under him.

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The baby screamed and Matthews struck her on the bottom with an open palm to stop her from crying. His actions caused the baby to receive a spiral fracture to the right femur and her leg was put in a cast.

Judge Saunders said Matthews had been charged "at the level of recklessness as opposed to intent".

He noted although Matthews had treated the child "very roughly", his actions had stemmed from anger or annoyance at the child, not malice. He was sentenced to two years and six months in jail, and afforded six months credit in recognition of his guilty plea.

Matthews appealed to the High Court in Christchurch that Saunders had enforced a starting point that was "too high", resulting in an "excessive" sentence. His lawyer emphasised the incident was an "isolated one which was not part of any pattern of violence" and said it had only happened as result of Matthews being "temporarily distracted".

In a decision released on Thursday, Justice Cameron Mander said it was an "entirely deliberate action which involved an excessive and unacceptable level of force".

He said the nature of the act could be likened to where an offender, in a state of frustration, grabs, shakes or squeezes a baby, recklessly causing harm.

Mander found that imprisonment of two years "would have been within range", but quashed the original sentence and substituted with a term of one year and seven months.