The convictions of three people for female genital mutilation in NSW will stand after Australia's highest court ruled the practice is illegal in all its various forms.

The ruling comes four years after the trio was convicted by a jury of the FGM of two primary school-aged sisters in two separate ceremonies the occurred between October 2009 and 2012.

The two girls' mother, who cannot be named for legal reasons, and midwife Kubra Magennis were each handed a minimum 11-month custodial sentence, to be served by way of home detention, in 2016 after they were found guilty of "mutilating the clitoris" of the girls.

Dawoodi Bohra sect community leader Shabbir Mohammedbhai Vaziri was also found guilty of being an accessory to the girl's mutilation and was also sentenced to a minimum of 11 months' imprisonment.

The two sisters suffered through ceremonies known as "khatna" which the High Court says involves causing injury to a young girl's clitoris by cutting or nicking it.

Charges were laid after the girls told a female police officer and a social worker they had been subjected to "khatna".

The mother and Magennis admitted they were present in a room with the girls for a "symbolic ceremony" - however they say the ceremony involved Magennis placing forceps on the girls' vulva's, not clitorises, without cutting them.

But in an electronically recorded interview with police, the elder girl described the tool used on her as something that "looked a bit like scissors".

She said during her "khatna" she was told to close her eyes and felt "a bit of pain and then a weird sort of feeling" in her private parts. She said the pain had felt mostly like a cut.

During the ceremony, the girl said there had been four women, including her mother, surrounding her to "calm her down".

"My mum tells me not to go around telling everyone (about it) much," she told investigators.

Covertly recorded evidence caught the mother admonishing her daughters following their interviews with police for revealing "a big secret".

The trio's convictions were quashed after the NSW Court of Criminal Appeal found that the word "mutilate" meant some imperfection or irreparable damage had to have been caused.

A majority of the High Court on Wednesday ruled the NSW Court of Criminal Appeal erred when it quashed the convictions, saying the phrase "otherwise mutilates" in the law banning female genital mutilation does indeed encompass someone simply cutting or nicking a girl's clitoris.

"This broader construction ... would best promote the purpose or object of prohibiting such procedures generally," Chief Justice Susan Kiefel and Justice Patrick Keane said in a joint judgment.

"On the Court of Criminal Appeal's construction, it may be taken as intended that even if a child might suffer a painful and distressing experience, no offence is committed unless some defect of damage is apparent."