Blenheim mum Donella Knox jailed for murdering intellectually disabled daughter

A Blenheim mother has been sentenced to four years in jail for murdering her severely disabled daughter.

Donella Knox gave her daughter Ruby sedatives then suffocated her in May last year, prompting what a High Court judge called a "once in a generation case".

She was sentenced at the High Court in Wellington in December, but court suppressions meant it could only be reported now.

Knox cared for her daughter full-time and on her own for 20 years.

Ruby was severely autistic and severely intellectually disabled. She couldn't communicate, was violent and needed constant care.

"Most of humanity isn't aware of how it feels to feel so trapped and so helpless," Knox's lawyer Simon Shamy said.

At age 10, Ruby already weighed 74 kg. While she grew physically, her mental development stalled and she behaved like a toddler.

At Knox's sentencing in December, Justice Joe Williams acknowledged she faced a difficult, stressful and dangerous job caring for her daughter.

"She would pull hair, head butt, scratch other children," Justice Williams said. "She would attack animals and you."

Ruby Knox autistic
Ruby as a child (File)

On May 16th last year, Knox could no longer cope. 

She gave Ruby 20 sedatives, then held her hand over her mouth until she stopped breathing. 

She then drove to the Blenheim police station and told police what she'd done.

In the six months before the murder Knox had become increasingly stressed, sleep deprived and desperate.

"A feeling came over you, and you thought this has to stop, now," Justice Williams said.

Health professionals said she had been adequately supported - and had received more assistance than anyone in Marlborough.

But Justice Williams accepted Knox felt let down by the health system and that she had run out of options.

Knox had previously had suicidal thoughts, once telling current affairs show 20/20 "at times it would have just been easier to pop off... just me and Ruby."

Blenheim rallied to support Knox after her arrest - her love for her daughter was never questioned.

"You cared for Ruby constantly, unselfishly, and unconditionally," Justice Williams acknowledged.

He likened the pair to having an abusive relationship - Knox was the punching bag for a child who didn't and couldn't know any better.