Mental illness behind Sydney dad killing

Luke Costin

A Sydney man who brutally killed his elderly father with a hammer and sharp items is not guilty of murder by way of mental illness, a judge has found.

Steve Bojnovic, then 39, waited on the couch with scissors before fatally attacking his father Slavko Bojnovic, 87, in the Liverpool home they shared in September 2017.

The only child repeatedly struck his dad over the head with a mallet-style hammer before cutting his throat with a knife.

"The accused later recalled he just kept going and going," NSW Supreme Court Justice Richard Cavanagh SC said on Wednesday, before accepting the killer's special plea of not guilty.

The special plea of not guilty by reason of mental illness - for which the onus of proof is upon the accused - was earlier accepted by the Crown.

Two medical experts had determined Bojnovic was suffering from acute psychosis at the time of the killing as a product of underlying schizophrenia, Justice Cavanagh said.

Symptoms included delusions of persecution and a complete lack of insight into his actions.

"He lacked the capacity to know his actions were wrong," the judge said.

Bojnovic, whose condition has improved since his arrest in September 2017, will remain in custody for the foreseeable future.

His future will be determined by the Mental Health Review Tribunal.