Victoria's courts have been handing out inadequate sentences for child abuse in families for decades, partly because of a "remarkable" precedent set in a 1986 incest case, the High Court has found.

Key points: High Court orders Victoria's Court of Appeal to re-sentence man who was jailed for three and a half years for incest

High Court orders Victoria's Court of Appeal to re-sentence man who was jailed for three and a half years for incest Victims of Crime Commissioner says it proves Victorian judiciary is too lenient

Victims of Crime Commissioner says it proves Victorian judiciary is too lenient Attorney-General says decision gives Victorian courts more freedom to sentence based on "gravity of offending"

The court made the finding when it yesterday ordered the Victorian Court of Appeal to re-sentence a man who had sex with his de facto partner's daughter when she was 13.

The man was given a three-and-a-half-year jail term for the offence of incest, but the High Court found that was "manifestly too low".

The man had sex with the girl in her mother's bed while her mother was in the shower, causing the girl to fall pregnant, in 2013.

The girl told her mother the pregnancy was the result of sex with a male schoolfriend, and the man went along with the lie, which led to the family moving to rural Victoria.

After the man was sentenced, the Director of Public Prosecutions appealed, arguing the sentence was "manifestly inadequate" and the pregnancy was an aggravating factor.

Victoria's Court of Appeal found that even though the sentence was "extremely lenient", it was within the range set by previous rulings and therefore should not be changed.

But the court also concluded that Victoria's track record on sentencing for incest was "demonstrably inadequate".

It found some past penalties did not reflect the seriousness of the crime, and jail terms should be increased for those incest offences deemed to be of medium seriousness.

The High Court yesterday backed the Court of Appeal's assessment, but ruled it should have gone further and re-sentenced the man.

"Having reached a conclusion that current sentences were so manifestly [inadequate] there was no good reason for the Court of Appeal not to correct the effect of the error," the High Court ruling said.

It found that sentencing for incest in Victoria had been at an "anomalously low level" for 30 years, largely because of the "gravitational pull" of a precedent set in 1986.

In that case, a man who had sex with his 14-year-old stepdaughter "despite her screams and struggles" was sentenced to six years' jail. The sentence was reduced to four-and-a-half years on appeal.

'If a sentence is wrong, the court should fix it'

The state's Victims of Crime Commissioner, Greg Davies, says the ruling confirms sentencing for many serious crimes needs to be toughened up in Victoria.

Mr Davies said the ruling confirmed there were far too many lenient sentences in the state. ( ABC News: James Oaten )

"I think this is a very strong message from the highest court in the land that sentencing in Victoria has lost its way and there are far too many lenient sentences," Mr Davies told AM.

"No judicial officer, whatever rank, likes to see their decision overturned on appeal so hopefully it will have a positive impact."

The Chair of the Victorian Sentencing Advisory Council said the High Court had essentially found Victoria's appeal court had made an error by placing too much weight on "current sentencing practices".

"[The ruling] basically says, if a sentence is wrong, the court should fix it," Arie Freiberg said.

He said the judgment won't change the range of factors courts are obliged to weigh up in sentencing offenders in Victoria.

"It looks at the maximum penalty, it looks at the offender's culpability, it looks at their prior record, it looks at aggravating and mitigating circumstances, it looks at the effect of the crime on the victim, and in Victoria, it has to look at current sentencing practices to try to ensure consistency in sentencing between individual judges."

The state's shadow attorney-general John Pesutto welcomed the High Court's decision, but said it wouldn't replace the need for legislative change.

"I think a decision like this couldn't have come soon enough because we've been calling on the Government to address this very issue, of whether sentencing practices dominate too heavily sentencing outcomes."

Sentencing practice 'a guide, not constraint'

The Victorian Government has passed legislation to introduce a system of "standard sentences" to help guide judges in deciding very serious cases.

But Mr Pesutto said if the Coalition wins next year's state election, it would introduce tougher rules.

"For a range of very violent offences, Victoria's most violent and serious sexual cases, there are 11 of them we've announced so far, we will introduce mandatory minimum sentences, which the High Court has said is a perfectly legitimate way to go."

But Professor Freiberg believes the High Court's ruling reinforced the case against mandatory minimum sentences.

"The High Court has emphasised the importance of individualised judgments, above that consistency in sentencing, where it's reflected in legislation applicable in Victoria that a court should take into account current sentencing practices."

In a statement, Victorian Attorney-General Martin Pakula said "the decision effectively means that current sentencing practice should be a guide, rather than a constraint".

"As such, it will ensure Victorian courts have more freedom to impose sentences that reflect the gravity of offending.

"That is something I welcome."