The NSW Minister for Families, Communities and Disability Services has parental responsibility for the 17-year-old, referred to under the court's pseudonym of Joseph, as he is unable to care for himself due to injuries sustained in a car crash as a baby.

The department sought an urgent application for the court to determine a statutory will for Joseph as he was awarded $12 million in damages in March via a lawsuit stemming from the crash and he will soon turn 18.

Supreme Court Justice Trish Henry accepted evidence from a clinical neuropsychologist that Joseph did not have the capacity to manage his financial affairs and was reported as having no understanding of the value of money.

"While he is able to speak and learn practical skills, Joseph requires supervision and assistance with all aspects of daily life," Justice Henry said.

Joseph's treating psychiatrist reported to the court that he had the intellectual capacity to understand what a will was and to express his intentions in respect of a will.

However, the psychiatrist stated it was her opinion that Joseph lacked sufficient ability to understand any obligation on him in respect of any people that may be included or excluded from benefitting from his estate.

"I also had a high degree of satisfaction that Joseph does not have the capacity to understand the real value of his estate, in terms of the real value of $12 million," Justice Henry said.

As a child, Joseph was involved in a car accident in which he sustained a severe traumatic brain injury, significantly affecting his cognitive development.

He was unrestrained in the car at the time of the crash and his mother's former partner was driving while drug-affected.

Joseph was placed into the care of his grandmother for six years until she was no longer able to provide full-time care due to his increasingly violent behaviour.

The NSW Minister for Families has had parental responsibility for Joseph on a permanent basis since 2012 after he was taken into care interstate following the car crash.

Since 2016, Joseph has lived in a residential therapeutic placement run by a non-profit that provides residential accommodation to young people who have complex support needs.

Joseph will remain at the placement at the non-profit until a property can be purchased and set up for him for one-on-one 24-hour care and support using funds from his settlement, which has $11,660,000 remaining.

The court heard that Joseph's caseworker at the NSW Department of Communities and Justice had spoken to him about creating a will.

Joseph told the caseworker that he was "bit unsure" whether to include his mother and step-father in his will but agreed, when prompted, that he would include any future long-term partner and children he might have.

Justice Henry ruled earlier this month that there was satisfactory evidence that Joseph was incapable of making a will under the Succession Act.

Under the Succession Act, Joseph's entire estate would have gone to his mother in the event he died without leaving a will.

Justice Henry found that did not align with Joseph's wishes and the fact that he spent his formative years with his grandmother and sister.

Justice Henry ordered a statutory will that would leave 25 per cent of Joseph's estate to any children he might have in the future, 25 per cent to any spouse, 25 per cent to his grandmother, 15 per cent to his sister or her children, and 10 per cent to a discretionary trust for his mother and step-father.

A charitable gift of $5000 to the NSW Rural Fire Service was also included as Joseph had engaged in volunteer work with the organisation.