Hayden Leigh Kidd, 22, initially faced a jury trial at the Supreme Court in Ballarat, accused of murdering Shirley Kidd at her Darley home west of Melbourne.

Prosecutors told the jury he attacked his grandmother on the evening of May 28, 2022, striking her head and torso with a garden mattock in the family's garage.

Two psychiatrists agreed that Kidd was in an altered mental state at the time of the attack, most likely caused by a cannabis-induced psychosis, the jury was told.

Kidd did not deny killing his grandmother, instead arguing he was acting in self-defence because he genuinely believed she was his abusive former stepfather.

The trial ran for eight days in Ballarat, with evidence called from the Kidd family, police officers and experts.

But on Friday, both the defence and prosecution applied to have the jury discharged on the charge of murder, with a fresh charge of manslaughter laid against Kidd.

The 22-year-old pleaded guilty to manslaughter and will return to the court in June for a pre-sentence hearing.

Australian Associated Press