Corporal Dominic Abelen, 30, was a member of the 2nd/1st Battalion Royal New Zealand Infantry Regiment (RNZIR) based in Burnham, near Christchurch.
The soldier had a decade of experience with the NZDF, but was on leave without pay from the army when he fought and died in Ukraine.
His father Bryce issued a statement through the NZDF saying the 30-year-old had not told his family as "he knew we would talk him out of it".
"He also knew the risks of going there but still went to fight for them. That is Dominic, always thinking of helping others," he said.
"He leaves a massive hole in all our hearts."
Defence Minister Peeni Henare expressed his sorrow over the incident, confirming the NZDF was supporting the man's family.
However, Mr Abelen is being treated by the New Zealand government as a citizen, not a soldier, given he travelled to Ukraine against orders and travel advice.
"I consider all of our NZDF personnel heroes. They are ambassadors and great leaders for our community and our country," Mr Henare said.
"The entire situation is a complicated one.
"(Soldiers) know the expectations from the NZDF and this government is that Ukraine is a no-go zone."
Foreign Minister Nanaia Mahuta said that, while Mr Abelen was not representing New Zealand or the NZDF, the government would assist with his repatriation.
"We will do everything that New Zealanders would expect us to do, as does the family, to work with the Ukrainian government to bring the citizen home," Ms Mahuta said.
Radio NZ reported Mr Abelen died in a firefight at dawn alongside international soldiers trying to re-take a trench network in the east of Ukraine.
Former NZDF commanding officer and Tauranga mayor Tenby Powell, who has relocated to Ukraine to assist with the humanitarian response, told Today FM he had been asked to bring Mr Abelen's body home.
"I feel very honoured to be asked to do that," he said, pausing to fight back tears.
"This never gets easier I can assure you, no matter how many times you hear about a soldier (dying).
"The intention is to drape him in a New Zealand flag and make sure he has all the care a soldier in the New Zealand Defence Force deserves on his way back."
Mr Abelen's siblings described him as "the most bold, brave and beautiful human with the quickest wit and freely given laugh".
"He sacrificed his life for others to have a life," they said.
New Zealand has offered military, logistical and financial aid to Ukraine in the wake of Russia's illegal and brutal invasion.
However, no on-duty troops have set foot on the ground.
New Zealand's most recent deployment was of 120 Kiwi troops to the UK to train Ukrainian fighters.
Former defence minister Wayne Mapp told Newstalk ZB around "15 to 20" New Zealanders may be in Ukraine among unathourised international fighting forces, and the government should keep track of them.
"This is a country we're supporting, we're fighting for their freedom. (Mr Abelen's) fellow soldiers will understand why he's done this," Mr Mapp said.
"Our embassy in Poland needs to know their names to support them in case of need."
Ms Mahuta said even though New Zealanders were advised against travelling to Ukraine, they could still declare their journey on the Safe Travel website.
6°C