A mentally ill man who sparked a 12 hour siege in Bunbury and held his chained up friend hostage with a speargun has been sentenced to four years and nine months in a mental health facility.
David Charles Batty, 54, held his friend captive at a park in the state's south-west in July 2015 - threatening to blow him up with a fake explosive he said was wrapped around his chest.
He was eventually detained by police after his victim managed to escape and police subdued Batty with bean-bag rounds.
During sentencing remarks on Wednesday, the Supreme Court heard how Batty was delusional and psychotic and believed police were following him.
He also believed his friend who he had captured was providing information to police about him.
Justice Michael Corboy said the case was "very difficult" and that Batty would be sentenced as if his delusions were true, as Batty believed them to be.
"However on this basis your beliefs could never have justified the serious nature of your offending," he said.
"You must have known your actions were very frightening for your victim.
"You also must have know how frightening your threats to blow your victim up would have been to him."
Batty is currently serving another sentence at Graylands Hospital for threatening to kill his father at Kings Park in 2012.
Once he has completed his sentence handed down on Wednesday, Justice Corboy said it was likely he would remain an involuntary patient at the Frankland Centre.
Batty's victim said in an impact statement to the court his enjoyment of life had been affected by the incident and that he suffered panic attacks and anxiety.
