Ross was Merritt's supervisor at Spotless Cleaning Services. Merritt had developed ill feeling towards Ross after she raised his lacklustre cleaning of some windows.
Merritt also had taken to parking his car in a disabled parking space at the company's Strathallan St premises.
The evening before the attack Ross gave Merritt a disciplinary letter detailing his perceived shortcomings, which would require a meeting with company bosses. In it, critically, was the threat of dismissal.
Early on the morning of the attack, dressed in dark clothing, he took a tack hammer to his workplace from his parents' Kaikorai home.
Ross attempted to flee, even trying to escape in her work van.
But Merritt pulled her from the van and continued his attack.
"She was struck repeatedly as she lay on the ground,'' Crown prosecutor Robin Bates said at today's sentencing hearing.
Ross sustained 15 blows to the head alone, and many others to her body.
Merritt, who has since been diagnosed as being on the autistic spectrum, sat emotionless as details of the attack were read to the court.
Bates said the murder was premeditated and "entirely disproportionate to the perceived slight''.
He accepted Merritt's "mild'' autism was a mitigating factor in his offending and the Crown sought a minimum non-parole period of 14-and-a-half years' jail.
However, defence lawyer Anne Stevens said Merritt's actions were significantly related to his mental disorder.
His "grossly disproportionate response'' to the employment letter was as a result of a misguided fight or flight response to what he perceived as a "sudden, almost catastrophic invasion of personal space'' tied to his autistic view of the world.
"Until this tragedy occurred Mr Merritt was undiagnosed,'' Stevens said.
Merritt also received his first strike under the three strikes warning legislation.
He had not previously appeared before the court.
