NSW Deputy State Coroner Harriet Grahame said the nine-month-old girl, known as Baby Q, "fell through the cracks" despite being known to child protection services in three states.

Baby Q died in November 2018 after being thrown into the Tweed River in northern NSW by her father.

A five-day inquest into the baby's death held in December 2023 heard both of the baby's parents had been diagnosed with severe mental illness and were untreated.

Around the time of the death, the family was homeless and sleeping rough in parks.

Ms Grahame said multiple failures had led to Baby Q's death when she handed down her findings in Tamworth on Thursday.

She found the risk of some kind of harm occurring to Baby Q was entirely predictable and inevitable unless there had been significant intervention by support agencies with the family.

"How she fell through the cracks is a matter of considerable concern to me," Ms Grahame said.

"While I accept that no single person had all the relevant information indicating the extent of the escalating risk involved, various people who had interacted with the family throughout 2018 should have understood that Baby Q was a child in urgent need of protection."

Counsel assisting Donna Ward SC told the inquest the father had reported hallucinations telling him to abduct and kill a baby.

On the day of Baby Q's death, the father tried to give the infant away to passers-by twice.

He later threw the baby into the Tweed River during a psychotic episode.

The baby's body was later found on a Gold Coast beach but her cause of death has not been determined.

The father was found not guilty of murder in 2020 on the grounds of mental illness following a judge-alone trial.

Ms Grahame said the baby's family had been living transiently prior to the death, moving between Victoria, NSW and Queensland, and the infant was known to child protection services in all three states.

She found the family's lifestyle made it difficult for the multiple government agencies in NSW and Queensland to stay in contact and provide support.

"It is important to acknowledge at the outset that while homelessness was one significant issue the family faced, their difficulties were much more complex," she said.

The coroner's recommendations are set to cover the sharing of information between government agencies across jurisdictions and the relationship between police and child safety organisations.

The recommendations are set to be published later on Thursday.

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Australian Associated Press