Judge Garry Neilson, from the district court in the state of New South Wales, likened incest to homosexuality, which was once regarded as criminal and “unnatural” but is now widely accepted.

He said incest was now only a crime because it may lead to abnormalities in offspring but this rationale was increasingly irrelevant because of the availability of contraception and abortion.

“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,” the judge said.