An Army corporal who bashed a lower-ranked soldier over a cheeky remark has today had his stripes removed and been thrown in army jail for 28 days.

Decorated Afghanistan war veteran Corporal Daniel Turua was investigated by military seniors after the assault during training exercise Sari Bair at Waiouru Military Camp in central North Island last September.

The 32-year-old junior non-commissioned officer serving with the Royal New Zealand Infantry Regiment 2nd/1st Battalion admitted four charges at a court martial at Burnham Military Camp, 30kms south of Christchurch today.

Turua became outraged when a private on the training exercise returned a lost beanie on September 8 and made a cheeky comment to his senior officer.

The junior NCO with more than 13 years' Army experience, who once saved the life of a sergeant while on exercise in Singapore, felt disrespected and embarrassed in front of his colleagues.

So, the next day, he told the private to wait in a training hut away from the rest of platoon.

Turua soon stormed in, grabbing him by the throat. He pinned him against wall and punched him in the head and face, the court martial heard.

The private suffered a cut above his left eye, a cut to his nose, and marks and bruising on his face and head.

After the assault, Turua called the remainder of the platoon over before bringing the private outside.

Turua told him to get on his "f..... knees and explain what happens when you disrespect a NCO".

The corporal then proceeded to threaten the rest of the troops: "This goes for any one of you mother......s. If you do this, I'll put you in an ambulance".

A military investigation was launched, which Turua co-operated with.

This morning, Turua admitted charges laid under the Armed Forces Discipline Act 1971 of assault, ill-treatment, and using threatening language.

The private, who has name suppression, said he's been "stressed out" since the incident, which left him with blurred vision and sleep problems.

He was medically-downgraded which caused him to miss a US deployment – something he'd trained hard all year for.

Army prosecutor Major Robert Goguel said the humiliation of a subordinate – by parading him in front of his comrades - caused a greater "erosion of military discipline" than the assault itself.

Given his seniority and experience, Turua should've known better, Goguel said.

An ashamed and remorseful Turua faced his victim today.

"Bro, from the bottom of my heart, I apologise for what I done that day," he said.

Turua had been "battling demons" at the time, and six months earlier, had reached out to a social worker and been placed on a stopping violence programme.

Since the incident, Turua says he's learned to show compassion, respect, and understanding, and that "having the courage to walk away is the right thing to do".

He's vowed to turn his life around and help stamp out violence in the armed forces.

Turua is now involved in a corporals' development team which is designed to identify issues in the battalion and develop solutions. He has a "passion and desire to bring situations like this to light".

"I can't undo the past but I'm trying to fix the future," he said.

It had been a "considerable fall from grace", defence counsel Elizabeth Bulger said. Turua, who lost comrades in Afghanistan, cannot believe he'd put his beloved career in jeopardy, she added.

Judge Heemi Taumaunu said sentencing must hold the senior soldier accountable for the harm he's caused, not just to the victim, but also to the wider military community.

His "bullying behavior" while in uniform was an abuse of trust and misuse of rank, which undermined respect for senior officers.

Although the assault "may or may not have been part of the culture you were in at the time", Judge Taumaunu said there was nothing to suggest it was anything other than a one-off incident.

The court martial sentenced Turua to 28 days detention in the Services Correctional Establishment (SCE) facility at Burnham.

The detainment means that he is automatically demoted two ranks, from corporal to private, and while he's inside, will only be on half-pay.

The judge concluded that it'll be up to his Army bosses if and when he can return to his rank.