An Australian judge will be investigated for his "appalling" comments on incest, in which he reportedly said sex between siblings was gaining social acceptance just like homosexuality, authorities said Friday.

New South Wales state Attorney-General Brad Hazzard said he was "extremely concerned" about District Court judge Garry Neilson's alleged comments in a case where a man was accused of repeatedly raping his younger sister, which were reported in the Sydney Morning Herald.

"In my view the community would be rightly appalled at his reported comments," Hazzard said in a statement.

"Incest is completely reprehensible, unacceptable, disgusting and criminal."

Neilson was quoted as saying communities might no longer see sex between siblings as "unnatural" or "taboo", likening a change in mindsets to how homosexuality was now socially accepted despite being criminalised in the past.

"A jury might find nothing untoward in the advance of a brother towards his sister once she had sexually matured, had sexual relationships with other men and was now 'available', not having (a) sexual partner," Neilson reportedly said.

Hazzard said he had referred Neilson to the Judicial Commission of New South Wales, which investigates complaints. He also requested the District Court suspend the judge from criminal trials until the probe was completed.

In his case comments, Neilson reportedly added that the "only reason" incest remained a crime was due to the risk of genetic abnormalities in children born through such a relationship.

"But even that falls away to an extent (because) there is such ease of contraception and ready access to abortion," he said.

The 58-year-old man accused in the case earlier pleaded guilty to sexually assaulting his sister when she was 10 to 11 years old in the 1970s, but not guilty to separate charges when she was 18, the Herald said.

The NSW prosecutor has reportedly requested that the ongoing case be transferred to another judge due to what she said was Neilson's "misogynistic" attitude towards the alleged victim.

In a separate case reported in Sydney's Daily Telegraph, Neilson was said to have given an incestuous rapist a lighter sentence in 2011 because he did not ejaculate inside his teenage niece.

The appeal judges reviewing that case last year criticised the stance, saying "it is difficult to see how that was a matter which could have been considered to reduce the objective seriousness of this offence in any real way".

Cathy Kezelman, president of support group Adults Surviving Child Abuse, said Neilson's alleged comments were "beyond belief" and such cases seriously damaged children.

"There are still a lot of myths, still a lot of entrenched, very damaging beliefs and that's why we need to speak out about it... so we're not hearing these outrageous statements," she told Fairfax radio.