Three people charged with female genital mutilation offences against two primary school-aged sisters could face further punishment after the High Court ruled the New South Wales Court of Criminal Appeal erred in quashing their convictions.

Key points: The High Court rules female genital mutilation is illegal in its various forms, including where there is no visible physical damage

The High Court rules female genital mutilation is illegal in its various forms, including where there is no visible physical damage The appeal stemmed from Australia's first prosecution on genital mutilation

The appeal stemmed from Australia's first prosecution on genital mutilation The NSW Court of Appeal will now decide if the original jury verdict was reasonable

Warning: This story contains graphic details that may confront some readers.

In 2015, the girls' mother and a former nurse Kubra Magennis were found guilty of two counts each of breaching the ban on female genital mutilation in NSW.

Dawoodi Bohra community leader Shabbir Mohammedbhai Vaziri was convicted of being an accessory.

All were sentenced to 15 months in jail but, while the women were allowed to spend the sentence out of custody, Mr Vaziri was jailed.

It was Australia's first female genital mutilation prosecution.

But the charges were quashed by the NSW Court of Criminal Appeal last year, after the trio argued the ceremony was only ritualistic and new evidence showed there was no visible physical damage to either girl.

Today, in a divided ruling, the High Court found the law did cover the circumstances in the case and that it was meant to criminalise the practice in its various forms.

'Skin only sniffs the steel' in ceremony

The religious ceremony of Khatna is said to involve a girl's clitoris being nicked or cut in the presence of elders.

The girls were believed to be aged six or seven when the ceremony was carried out at their homes, one in Wollongong and the other in Sydney, between 2009 and 2012.

At the original trial the eldest child had given evidence describing Khatna, saying "they give a little cut … in your private part" using a tool similar to a pair of scissors.

However, Ms Magennis said the ceremony had used forceps, not a blade, and the "symbolic" form of Khatna involved a "ceremony of touching the edge of the genital area … allowing the skin to sniff the steel".

Shabbir Mohammedbhai Vaziri had been handed a maximum 15-month sentence. ( AAP: Dan Himbrechts )

After the trio were convicted, a medical examination showed the tip of the clitoral head was visible in both girls, allowing them to argue successfully that the genitals were not mutilated.

The convictions were set aside but prosecutors sought an appeal to Australia's top court, arguing the actions of the three had still breached the NSW law.

Today the majority of the High Court bench found in the NSW prosecution's favour, deeming the trio's actions illegal.

Retrial may cause girls psychological harm: Chief Justice

The case has been referred back to the NSW Court of Criminal Appeal for further consideration about whether the jury's verdict was unreasonable.

In a joint judgement with Justice Patrick Keane, Chief Justice Susan Kiefel said normally a new trial would be ordered, but in this case that may not be appropriate.

"[The victims] C1 and C2 were children when they were interviewed by police and when they gave evidence at a trial which took place in 2015," she said.

"The trial judge, in considering whether C1 and C2 were compellable to give evidence against their mother, accepted that there was a likelihood that psychological harm might be caused to them.

"There could be little doubt that a second trial would compound that distress."

Justices Virginia Bell and Stephen Gageler differed from the rest of the court taking a narrower view of the offence and argued that the Court of Criminal Appeal made the correct decision.

"The Court of Appeal was right to hold that superficial tissue damage, which leaves not physical scarring and which on medical examination is not shown to have caused any damage to the skin or nerve tissue, is not in law capable of amounting to mutilation," they said.

The case has been listed for a call over in the NSW Court of Criminal Appeal next week.