The off-duty Kiwi soldier killed in Ukraine never told his family he was going off to fight, his father said today while paying tribute to a "kind, gentle man".

Dominic Bryce Abelen, 30, was on leave away from the New Zealand Defence Force (NZDF) and was not on active duty when he was killed in a firefight trying to re-take an enemy trench.

The corporal with 10 years' army service was based at Burnham Military Camp outside Christchurch with the 2nd/1st Battalion Royal New Zealand Regiment.

He had been fighting with Ukraine's International Legion on the front line in the east of the war-torn country.

Dominic Abelen, from the Royal NZ Infantry Regiment's 2/1 Battalion, killed in Ukraine. Photo / Facebook

This afternoon, his father Bryce Abelen spoke of his son who he says was "always thinking of helping others".

"Dominic was in the NZ Army for 10 years but never got to fight for his country," he said.

"He didn't tell us he was going to Ukraine until he was there. He knew we would talk him out of it. He also knew the risks of going there but still went to fight for them.

"We understand why he went and hold no grudge against the NZ or Ukraine Army and fully support what he did."

• 'Strong, hardcore, handsome but extremely humble': NZ soldier killed in Ukraine is named
• Kiwi soldier killed in Ukraine: Details emerge of firefight

The grieving father said Abelen loved being in the army and the lifelong friends he made there.

"There has been relentless support from half the Burnham Military Camp that knew him, as well as personnel from overseas," Bryce Abelen said.

"A kind, gentle man with the most infectious smile that you couldn't help but smile back.

"He had a love of the outdoors and did a lot of tramping. He spent his holidays travelling around New Zealand and the world.

"I'm extremely proud of my children and especially Dominic for standing up and doing what he thought was right."

His death leaves a "massive hole in all our hearts", he said.

Corporal Dominic Abelen. Photo / NZDF

Abelen's sister and brothers also released a collection of statements that recalled "the most amazing human being" and "the buffest Abelen with the biggest heart to match".

"I am so proud to call you my brother, I knew you always wanted to risk your life for what you believe in. You proved that so I won't be sad that you are gone but proud that you lived the life of a hero."

Another said: "Dominic, my brother is more than just another soldier, he is the most bold, brave and beautiful human with the quickest wit and freely given laugh. Our hearts are broken. He sacrificed his life for others to have a life.

"He was a standard high achiever in all aspects of life. Not only was he my big brother, my role model, my support, but he was for many others too. He took the army brotherhood to a new level, showing it's not about how hard you are but how hard you work. Actions speak louder than words and Dom had said more than enough.

"He was a career-driven man, a reliable brother who always had our backs, a favourite and very much loved uncle to a nephew who is waiting for him to come home."

Acting Chief of Army Brigadier Rose King said the New Zealand Army's thoughts were with Abelen's whānau, friends and colleagues.

"Any loss of one of our whānau is deeply felt across the New Zealand Defence Force. We are concentrating our efforts on supporting Corporal Abelen's loved ones and our personnel as they grieve," she said.

The NZDF is supporting consular officials in respect of possible options regarding formal identification and repatriation, noting the very difficult circumstances of doing so in a conflict zone.

Commanding Officer of 2/1 RNZIR Lieutenant Colonel Cory Neale said Corporal Abelen was a well-liked and respected soldier who had deployed to Iraq during his time in service.

"Corporal Abelen was a quality junior non-commissioned officer, a trusted go-to member of his company and an absolute character who genuinely cared for his soldiers and friends. He will be missed, but also remembered long into the future," Neale said.

The NZDF says it is unable to comment on the circumstances of the incident itself as Abelen was not on active duty.

But it said any uniformed personnel are required to notify their chain of command of international travel plans when taking leave without pay, however are not required to check in routinely.

The NZDF is still inquiring into the circumstances of Abelen's case as his leave request did not include plans to travel to Ukraine.

It's not clear how long he had been in the country but he had been with other Kiwis and already been engaged in other "contacts" with the enemy.

The NZDF says it is not known how many NZDF personnel on leave without pay may be in Ukraine - and that no personnel have been approved by NZDF to enter the country.

The organisation also doesn't know how many ex-serving personnel may have travelled to Ukraine and when someone leaves the Defence Force, the NZDF does not track their movements.

Sources have told the Herald that Abelen was involved in a joint operation targeting the re-taking of a frontline trench network.

During an assault at dawn, Abelen was killed in firefight with Russian forces and died instantly, sources say.

An American fighter with the international troops also died.

They also claimed many casualties on the other side.

One former NZDF soldier fighting alongside Abelen with Ukraine's International Legion has paid tribute to the fallen comrade.

"The bro was anything but scenery. Strong, hardcore, handsome but extremely humble," he wrote online.

"Best believe he died doing what he loved and was extremely good at.

"We will miss you brother. So much. You have left a hole that we are feeling and we could never hope to ever fill."

He called him a "warrior until the end".

Others have spoken of their shock at losing a "solid operator" and a brother of the famed 2nd/1st battalion.

Former soldier and No Duff Charitable Trust co-founder Aaron Wood today said it was only a matter of time before a New Zealander was killed in the fierce fighting.

"If anything, it's surprising it hasn't happened sooner, and more often," said Wood who likened the fighting there to "every bad nightmare story that came out of the eastern front circa 1944/45".

"When people engage in that, you're counting the days," he said.

"While it is obviously very sad that he was killed, he knew what he was doing. I don't mean to sound cliched or callous, it's more of a pragmatic approach.

"He would've known the risks and had been there a while... and quickly figured out whether or not it was for him … and the fact that he stayed, he was comfortable with it."

Ukrainian President Volodymyr Zelenskyy announced the formation of the International Legion for the Territorial Defense of Ukraine on February 27 which called for foreign volunteers to join the fight against Russia under the Ukraine flag.

Within 10 days, it had reportedly received 20,000 requests to sign-up from people across more than 50 countries.

Wood has been in touch with several former NZDF personnel who have answered Ukraine's global call for help.

He says it would total around 100 people, with some being there for a brief period, others returning for multiple trips, and a few who have been there for the duration of the war.

The number at "the tip of the spear" like Abelen numbered "in the very low tens", Wood said, along with doctors, medics, and others in support.

Each individual has their own reasons for going, Wood said, ranging from those wanting to stand up against Russian aggression in what they see as a "righteous fight", to those servicepeople who trained for years but maybe never had the chance to put their skills or mettle into action, and the many "at a loss" after leaving the NZDF and struggle with the transition to civvy-street.

Earlier this month, the government announced it was sending a further 120 NZDF personnel to Britain to help train Ukraine soldiers as part of an international effort to help Ukraine continue to defend itself against Russia's illegal war.

It followed a completed deployment of 30 NZDF personnel in May to train Ukrainian military personnel in operating artillery.

"New Zealand has been clear that we will continue to answer the call of Ukraine for practical support as they defend their homeland and people against Russia's unjustified invasion," Prime Minister Jacinda Ardern said on August 15.

"We know that one of the highest priorities for Ukraine right now, is to train its soldiers, and New Zealand is proud to stand in solidarity alongside a number of other countries to answer that call."

Defence Minister Peeni Henare expressed his condolences to the family, friends and colleagues of the Kiwi soldier.

"I have been advised the New Zealand Army are supporting the soldier's family through this difficult time," he said.