West Australian mum jailed for her baby's 'violent' death.

A West Australian woman who claims she still does not remember how her five-month-old son died has been jailed for 8 years over his death.

The five-month-old’s official cause of death was a traumatic brain injury, but he was found with traces of methamphetamine in his system, though the court heard that it did not contribute to his death.

Melissa Louise Bulloch, 27, admitted killing her five-month-old Lochlan in January 2014 after a police investigation that included installing recording devices in her home.

Melissa Louise Bulloch, 27, admitted the manslaughter of five-month-old Lachlan. Via Seven News.

At a sentencing hearing in WA's Supreme Court Justice John Chaney said that it was “obvious the baby had suffered significant violence and trauma" at the hands of his mother.

Justice Chaney also said there had been a "long delay" in Bulloch accepting responsibility for the death - and had tried to deflect blame onto others before her guilty plea.

The ABC reports that at an earlier court hearing Bulloch suggested her partner, his mother or a "good Samaritan", who had helped her when she ran out of fuel, could have been involved in the death of Lochlan.

Prosecutor Carmel Barbagallo described Bulloch’s abuse of her baby son, saying that during his short life he had cannabis smoke blown into his face, had been left in a car for long periods, was shaken and sworn at.

Bullock however, told the court she does not remember the exact cause of Lochlan's death.

The court heard she put Lochlan in his cot, and then spent a sleepless night smoking cannabis to come down from a prolonged meth binge. When she then checked on him at 7am she called Triple O. In one portion of the tapped emergency call she says “he’s got bruises all over his face and I don’t know where the hell they came from”.

It turned out that she was lying.

Police, who had tapped her phone, heard her admitting months later “basically, someone has shaken him to death."

The West Australian reports that in the months after his death Bulloch was heard by police saying several times that she was “sorry” for killing him.

"I'm sorry for killing you my sweetheart .... That freaked me out."

Lachlan was 5-months-old. Via Nine News.

In a letter to the judge, Bulloch said she could "acknowledge and take full responsibility for this tragedy" and her son "deserved to live, love, laugh and experience life".

While she could not "take back what happened", Bulloch said she would "forever live with this guilt, shame and despair".

She will be eligible for parole in 2020.