"Help me, help me, my baby's not breathing," screamed Ariah Roberts' mother minutes after an alleged violent and murderous attack that ended her short life.

The mother, who has name suppression, was "very hysterical" and was "shouting, screaming" on the night of August 22, prompting her landlords to race to the property to investigate.

Ariah, 2, was allegedly murdered by her stepfather Aaron Archer on August 22 last year.

Archer, 30, is currently on trial.

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He admits he caused the death of the child but says it was an accident and has pleaded not guilty to murder.

Ariah suffered "catastrophic" and multiple head injuries.

The injuries were allegedly inflicted in a 15-minute period while her mother went to the supermarket to buy dinner.

Ariah lived at a Mangawhai property with her mother and Archer.

2-year-old Ariah Roberts was allegedly killed in Mangawhai Heads in 2018. Photo / supplied
2-year-old Ariah Roberts was allegedly killed in Mangawhai Heads in 2018. Photo / supplied

It was a "bach-like" house situated in front of owner Helen and Stephen Taua's own home.

Today the jury in the High Court at Auckland heard from Helen Taua about what happened on the night Ariah was killed.

Helen Taua said she and her husband and daughter were watching television when they heard a commotion.

"I initially thought it was neighbours to the right ... realised it wasn't coming from there," she said.

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"I went out to have a look and saw [Ariah's mother] in the driveway, very hysterical, screaming 'help me, help me, my baby's not breathing'.

Stephen Taua came and took Ariah from Archer, who had been holding her.

His wife called 111 and they started to try and revive Ariah.

Crown Prosecutor Brian Dickey questioned Helen Taua about what Archer said about the incident on the night.

"Aaron talked about that he had dropped her ... As soon as we came in," she said.

"I'm sure he just said 'I dropped her ... she hit her head, I dropped her'.

"He said it a few times because he was really upset."

Helen Taua said Ariah was "lifeless".

"She was very pale ... she wasn't breathing, she had a small bruise on her forehead," she said.

"Aaron was really flustered he was just saying 'I don't know, I don't know, I don't know'."

She told the court Archer said several times: "I was swinging her around and I dropped her and she hit her head".

He did not elaborate on how or why he was swinging her.

Helen Taua also recalled Archer mentioning Ariah having a seizure or "shaking".

She and her husband tried frantically to save Ariah's life.

Under instruction from the 111 operator they put the child on the floor and started CPR.

Emergency services arrived soon after.

The Tauas went outside and supported Ariah's mother and Archer while paramedics worked on the child.

However, she was pronounced dead at the scene, her injuries too great to survive.

Helen Taua said she rented the property to Ariah's mother before she was in a relationship with Archer.

At the time the young mother was still with Ariah's birth father.

He left the address at some stage, and Archer moved in.

Helen Taua said she did not know Archer well, but sometimes spoke to him in the driveway.

Stephen Taua followed his wife to give evidence.

He said there was some "pretty hysterical screaming" coming from the rental property and "banging on the door".

"I went downstairs to see what the problem was and (Ariah's mother) said to me that Ariah wasn't breathing."

Stephen Taua raced to the bach and went in the front door into the lounge.

He could smell vomit but apart from a small bruise on Ariah's forehead, there were no obvious signs of serious injury.

"I wasn't really looking for any injuries, I was just trying to bring the baby back, to be honest," he told the court.

"While it was happening (Archer) was saying it was an accident and he was playing ... he said he was swinging her around and accidentally dropped her or she hit her head somewhere, something along those lines.

"I was just really focused on trying to bring baby back."

Stephen Taua said Archer appeared "traumatised" during the fracas.

"It must have been pretty tough for him too," he said.

Stephen Taua became emotional speaking about his efforts to save Ariah.

"I had to put my fingers down into her little mouth in case there was something blcoking," he said.

"No matter what I did, I just didn't get any response from her at all…"

Earlier today the jury were shown photographs Ariah lying dead in her own home after allegeldy being murdered by her stepfather.

During the trial they will also be shown graphic images from her post mortem examination which will show her head and body covered in bruises following the alleged violent assault where she was subject to multiple blows to the head.

After the Crown opened its case, outlining the allegation against Archer, the first witnesses were called - police and forensic who carried out the scene examinations.

The jury were shown still photographs and 3D imaging of the house where Ariah died so they could familiarise themselves with the property.

The images show the little girl lying dead on the floor, wrapped in an adult's pink bathrobe.

The trial continues.