Alo Ngata lay unmoving in a police cell with a spit hood covering his face after his arrest in 2018, and it was the first time the police officer who put on the hood had done it in a real-life situation.

She was the most junior officer and the last to arrive on the scene that fateful day, a coronial inquest into Ngata's death has heard.

Family members and supporters filled the public gallery of the Auckland District Courtroom today on the first day of the two-week inquest before Coroner Matthew Bates.

Ngata's mother Alofa Ngata led a moving karakia at its opening, saying the day had been "four years in waiting".

Her 29-year-old son was declared brain-dead and died in hospital days after his arrest in mid-2018.

Summarising the case at the start of the hearing, Coroner Bates said police had attended to an argument between Ngata and his partner the day before his arrest.

The attending officer saw Ngata showing "bizarre" behaviour and talking about seeing ghosts and spirits that day.

The next day, on July 1, 2018, Ngata was in a heated argument with his partner at home when he suddenly leapt to his feet, went out onto the road and attacked an elderly stranger, punching him and knocking him to the ground.

A resident who saw the incident unfold told the inquest he saw Ngata moving erratically, roaring at the sky, and jumping around like he was on drugs.

The man also heard Ngata's partner repeatedly yelling "Don't do this, I love you."

When the police Eagle helicopter arrived Ngata jumped up and down, flailing his arms in a defiant gesture at the helicopter, before turning to the elderly man and assaulting him once more, knocking the 81-year-old victim out.

Ngata then moved toward the police officers who had arrived at the scene.

Alo Ngata died in police custody in July 2018. Photo / Supplied

He was tasered but not subdued, and after a lengthy and violent struggle on the ground, was handcuffed, cable-tied at the legs, and fitted with a spit hood on his head.

The police officer who applied the hood had done it in training but that was her first time using it in a live situation, the inquest heard.

She made the decision to apply the hood because Ngata was spitting, including blood, on her. "He was agitated and aggressive," she said.

Five or six other officers were restraining Ngata while she tried to put the hood on him, she told the inquest. The first hood ripped in the struggle, and it was raining.

She found another hood and managed to put it on Ngata's head "correctly at the time," she said.

He was then lifted and placed in the back of a police van.

At the police cell, officers removed his restraints and left him lying prone, still wearing his spit hood, assuming he would remove it himself.

He did not move, and a short while later police removed the hood and found he had stopped breathing.

They started CPR and called emergency services, who arrived and took him to hospital.

Ngata never regained consciousness and was declared brain-dead two days later. He was kept on life support until July 4 to allow his parents to travel to New Zealand.

He died with his parents by his side, shortly after his life support was turned off.

A post-mortem found that Ngata died of brain damage, along with abnormal heart rhythm, restraint asphyxia, methamphetamine, an enlarged heart, and possibly suffocation due to the spit hood.

An investigation by the Independent Police Conduct Authority [IPCA] found in 2020 that police had failed in their duty of care to Ngata, but the force used to restrain him was appropriate.

The IPCA also found the initial use of the spit hood was correct, although its continued use should have been reassessed once he was lying prone in the van.

The inquest continues this afternoon.