A WOMAN who drowned her four-year-old son was yesterday found not guilty of murder by reason of mental illness.

Shan Shan Xu, 35, drowned her son Stephen in 2003 after leaving a note for her mother that said they would not be returning home to their western Sydney unit.

“We are not coming back home. Please look after yourself. Just treat it as if we’d had an accident. I’ve really had enough,” the note read.

Ms Xu was pulled from the Parramatta River, holding her dead son, by crew and passengers of a ferry.

She fled into an underground railway station yesterday to avoid waiting press photographers after the judgment was delivered in theNSW Supreme Court.

Shanghai-born Ms Xu came to Australia in the mid-1990s. She gave birth to Stephen in late 1998 after a brief liaison with a construction worker.

In late 2002, she began making suicide threats and in early 2003 she was admitted to an acute-psychiatric unit, the Caritas Centre. The hospital notes recorded that she had “thoughts of killing herself and her son … sees nothing wrong with this”.

Psychiatric staff described her as much brighter when she was released, a month before she killed her son. Only three weeks before the incident, a psychiatric registrar observed her as “co- operative, pleasant, smiling appropriately … there was no evidence of any psychotic symptoms”.

But on March 31, 2003, Ms Xu set out from Central Railway Station with her son and arrived at Meadowbank wharf about 2pm.

Closed-circuit television maintained by the Transport Authority showed she played in the park with Stephen until 6.41pm.

“There was no one else on the wharf. At the end of the ramp, she gathered Stephen in her arms and held him for some time. She then disappeared from the end of the wharf, still holding Stephen, as she stepped into the Parramatta River,” Justice David Kirby said in his judgment.

She was pulled from the river more than an hour later. Attempts to revive the boy failed.

Justice Kirby accepted the evidence of two forensic psychiatrists that Ms Xu was suffering a major depressive illness at the time.

Ms Xu has been living in the community with her mother for the past 10 months. Justice Kirby said under the circumstances he did not believe it appropriate to return Ms Xu to custody.

He ordered her to continue psychiatric treatment and to be periodically assessed by the Mental Health Review Tribunal.