In two run-down caravans and a couple of old tents on a farm in some of Australia's best wool-growing country, a dark secret of intergenerational incest and child abuse that has dominated the lives of 40 members of one family for decades has come to light.

In a community of around 2,000 people, three and a half hours' drive south-west of Sydney, genetic testing has shown 11 children in one family have parents who are related to each other, some closely. Questions are now being asked about why it took authorities so long to act on warnings that children were at risk.

Each child had suffered years of sexual abuse, perpetrated by brothers, sisters, cousins, fathers, an uncle and a grandfather over four generations.

The extended family first came to the attention of authorities in June 2010. Over the next two years, seven 'risk of significant harm reports' were issued, mostly relating to neglect, failure to seek necessary medical attention and failure to ensure the children attended school.

The makeshift camp where the family lived had no running water or toilets. The children did not know how to use toilet paper and some could not brush their teeth and ate with their hands. Dirt covered one of the caravan's cooking facilities and rotten vegetables lay in a fridge. A kangaroo slept on one of the children's beds.

Initially the family complied with a request in June 2012 from social services to improve their living conditions but a month later 12 children, all cousins, ranging in age from five to 15 years, were removed by police after the authorities concluded that they would be at risk of harm if they remained at the property.

The children were filthy, malnourished and barely able to speak. Some had moderate intellectual disabilities and could not be understood by social workers. Others had chronic hearing, sight and dental issues. One child, a nine year-old girl, was described as having 'dysmorphic' features. None had attended school regularly, if at all.

It soon emerged these children have been living a nightmare that went far beyond neglect. Assessed by psychologists and social workers, the children exhibited sexualised behaviour and reported that they had engaged in "inappropriate sexual conduct" with each other.

The intergenerational abuse is thought to date back 40 years. The grandparents of the children removed, Timothy and June Colt (pseudonyms given to them by the court), married in New Zealand in 1966. They had seven children, Rhonda, Betty, Cherry, Frank, Charlie, Paula and Martha (all pseudonyms). The family moved to Australia, living in various locations across four states.

Three of Timothy and June's daughters – Betty, 46, Rhonda, 47, and Martha, 33 – who lived together in squalor on the rural property, are mothers to 11 of the 12 children removed. The fourth mother is Betty's 30-year-old daughter, Raylene.

Only Rhonda's five-year-old daughter was found not to have been fathered by a blood relative.

Betty, Martha and Raylene each deny that their children are the progeny of incestuous relationships, despite the fact that genetic testing has proved otherwise

Three of the girls aged seven, eight and nine claimed that their uncle, Charlie Colt, was also their father. He lived on the property when the children were removed. The nine year-old also claimed Charlie Colt had sex with her.

It's believed that the family patriarch, Timothy Colt, who died in 2009, fathered children with one of his daughters and one of his granddaughters.

Five of the boys removed also admitted to torturing animals on the farm including puppies and cats. They also said they had mutilated the genitals of animals.

All of the removed children will remain in care until the age of 18. The case came to light after the children's court of New South Wales took the unusual step of publishing its judgment in the case.