The little girl in the red dress smiles for the camera, her plastic crown sparkling with fake jewels and bearing the words 'Happy Birthday', her eyes bright and excited.

She is surrounded by gifts and crouched beside her, wearing a matching necklace, her mother smiles too.

The pair look happy, close, loving - a normal little Kiwi girl with her mum posing for a birthday party photograph.

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Insane: The Kiwi killers found not guilty

Maggie Watson's mum not guilty by reason of insanity
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But what happened to this little girl a few months later was far from normal, loving or happy.

Her name was Maggie Renee Watson and in August last year she was killed in her Onehunga home by her mother Evelyn Kathleen Sen.

A post-mortem examination would later reveal Maggie's body contained more that 130 times an adult's dosage of mirtazapine.

Today, Sen was found not guilty of Maggie's murder by reason of insanity.

There is no denial that she killed the 4-year-old - but today in the High Court at Auckland, Justice Matthew Downs ruled she could not be held criminally culpable for her actions because she was legally insane at the time.

"She believed she and her daughter were posessed by demons," Justice Downs said.

She will be made a special patient and detained indefinitely at a forensic mental health facility.

Sen burst into tears when Justice Downs gave his decision.

'Maggie was the world to her'

Maggie was born in at Middlemore Hospital on December 28, 2010, to Sen and her then-partner.

Sen hailed from Johor Baru in Malaysia and her partner was English but they were living in Auckland when their only child was born.

Their relationship would not last and her partner would go back to the UK, leaving Sen to raise Maggie on her own.

In 2013 Sen and Maggie spent time living at a women's refuge in Auckland.

A woman who was sheltering there at the time told the Herald that Sen "adored" Maggie, who she described as a gorgeous child who was wise beyond her years.

"Maggie was the world to her," the woman said.

"She was a such a clever and bright girl. As a toddler she knew all the animals, all her shapes and colours, she was so clever and such a chubby little thing, a gorgeous girl."

The woman revealed Sen had a troubled relationship in the past and was terrified to be alone. She previously changed her name.

"We had offered for her to stay with us but she didn't want to be a burden," the woman said.

She saw Sen and Maggie at a local library two months before the little girl died.

"She was doing well and happy, just so happy."

Friends of 4-year-old Maggie gather outside her home on Moana Ave. Photo / Greg Bowker
Friends of 4-year-old Maggie gather outside her home on Moana Ave. Photo / Greg Bowker

'They told me it could not be an accident'

It was 4am on August 7, 2015, when the 111 call came in.

Sen had picked up the phone at her Moana Ave home and called the police.

Officers dispatched to the house found Sen with injuries described at the time as superficial.

Sen was rushed away for medical treatment. There was nothing they could do for Maggie, though, the little girl was dead.

Sen went into the care of mental health professionals as the police began to investigate what happened to Maggie.

A post-mortem examination was carried out but the results were inconclusive and police ordered a second round of testing.

Those results were much clearer - they showed police that Maggie had not died of natural causes. She had been killed by someone else.

While police remained tight-lipped on their investigation, Maggie's grandfather spoke out.

Clifford Sen told the Herald on Sunday that he had spoken to investigators who had informed him a homicide investigation had been launched after they received the toxicology results from the second post-mortem exam.

"I asked about the autopsy result and [police] told me the toxicology results had come in and they're now treating it as a homicide," he said.

"I asked, 'is it possible that Maggie could have accidentally ingested it herself' and [the police] said no; [the police] did not want to say what the actual chemical or drug was that was found."

The news was devastating for Clifford Sen, who was visited in Malaysia by his daughter and granddaughter not long before the death.

"I was quite taken aback ... they told me that it could not be an accident.

"That's very clear, that was the first question I asked, thinking it could be something edible and [police] said, no, Maggie couldn't have done it on her own.

"When we found out that Maggie didn't pass away naturally, that it was somebody who did it to her ... each step is getting more and more traumatic for us, especially my wife."

Clifford Sen described Maggie as an angel who was intelligent and inquisitive.

"She loved looking at pictures and being able to describe them," he recalled.

"She was so outward, she loved to talk about dinosaurs and the universe and things like that at her young age."

The 'girlie girl' with the constant smile

Maggie was cremated after a funeral at St Mark's Catholic Church in Pakuranga 12 days after emergency services were called to her home.

Clifford Sen and his wife, Pat, were unable to get to Auckland for the service but wrote a eulogy that was read by Father Emile Frische to the 50 or so mourners.

Maggie's mother was not there.

In their tribute her grandparents said Maggie was intelligent, compassionate and never shy.

"She could sense when you were sad or troubled, and would ask you if she could hug you.

"She would engage with strangers and smile and talk to anyone with ease.

"Why you had to leave us so soon I guess we will never understand. We will miss you tremendously."

Des and Kim Gilmore lived next door to Sen and Maggie and helped to organise the funeral.

Both spoke at the service about their "beloved little Maggie" and her constant smile.

The couple hosted Maggie's third birthday party and were extremely fond of the child, saying she was like "a beam of sunshine on a winter's day".

Des Gilmore remembered the little girl's love of the Disney movie Cars and said whenever he arrived home in his car she would come out to see him.

He said he would always regret not taking the youngster out for a drive.

"It breaks my heart knowing that she isn't going to get that opportunity to go and do that with me," he said.

His wife described Maggie as a "girlie girl".

"She was adorable. She loved Dora the Explorer, Happy Feet, and of course the latest craze Frozen."

"She adored her mum and her mum adored her," Kim Gilmore said.

After Maggie's death the Gilmore's also organised a community vigil on Moana Ave - a chance for her friends to remember her.

"... it has really shaken up the community in a few ways," Des Gilmore said at the time.

Among those at the vigil were Alosina Amosa and her mother Fialelei, who lived next door to Maggie.

"We always played together," said Alosina, 12.

Maggie Watson and her mother, Evelyn. Neighbour Kim Gilmour said Maggie
Maggie Watson and her mother, Evelyn. Neighbour Kim Gilmour said Maggie "adored her mum and her mum adored her". Photo / supplied

In February, police revealed that a 44-year-old woman had been charged with murdering Maggie.

She appeared in the Auckland District Court and was granted name suppression.

In July the suppression order was lifted and it could finally be revealed that Sen was charged with murdering her daughter.

Sen was set to go on trial in the High Court at Auckland next month, but today's ruling superseded that.