IT was an almost unbelievable conversation overheard between children at school: “My sister is pregnant and we don’t know which of my brothers is the father.”

It was more than enough for one of the teachers at the southern NSW high school to alert authorities, which led to the exposure of a feral family and the worst case of incest and sexual abuse ever uncovered in Australia.

What triggered the raids on the isolated farm by police and the Department of Family and Community Services has been revealed in court documents as Parramatta Local Court has ­ordered two of the family members to have DNA tests.

Detective Sergeant Kristy Hales, with the state crime command child sexual abuse squad, had asked the court for an order authorising the tests on 14 adults and two children as police ­continue to investigate the family, known by the pseudonym Colt, for charges of rape and sexual assaults.

The family’s reclusive lifestyle was uncovered last year when the Children’s Court took the unusual step of releasing its findings that led it to placing 12 children in care.

The raids found 38 adults and children living in filthy caravans and tin sheds without electricity, town water or any plumbing. They were the result of four generations from grandparents who were brother and sister.

Detective Sgt Hales affidavit to the court revealed that there were more than 12 risk of harm reports made about the family between August 3, 2011, and July 18, 2012, ­received by FACS “with minimal action taken”.

Finally there came the ­report from the teacher.

“This report stated that one of the Colt children was overheard saying at school to the other children ‘my sister is pregnant and we don’t know which of my brothers is the father’,” the affidavit showed. The court was told that the sister, given the pseudonym Tammy Colt, 29, gave birth to her baby Sally, who died within two months from a fatal genetic disorder known as Zellweger Syndrome. Sally was born “extremely dysmorphic” with the thick short neck and lowset ears typical of the syndrome.

The father was not named on the birth certificate but Tammy has since told counsellors she had been having a relationship for three years with a younger brother.

Genetic testing on the baby girl showed the genetic abnormalities existed as a result of incest and it is believed both parents are carriers of the genetic disease.

A doctor made an appointment for Tammy and her mother, Betty Colt, to see her to warn them of the dangers but neither of them turned up. Tammy has since been found living in a southern NSW caravan park and her other two daughters taken into care.

Betty Colt appeared yesterday in Moss Vale Local Court where her case was adjourned for sentence on August 18.

Originally published as How a school remark exposed incest