New Zealand has targeted sanctions against Russian oligarchs and their families, with 36 individuals added to the list overnight.
Ardern said reports of Ukraine civilians who had been killed, raped and severely wounded by Russian troops were "beyond reprehensible".
"Russia must answer to the world for what they've done. It goes without saying that these atrocities are against international law.
"New Zealand, alongside other Rome Statute parties, has already referred the situation to the International Criminal Court and we continue to support prosecutors in their investigations."
Ardern said every piece of evidence "points to the fact that there are war crimes being committed by Russia in Ukraine at the hands of the President Vladimir Putin".
"When it comes to the process of determining that is indeed what has occurred based on the evidence we've seen, that is something we are supporting a formal process around."
However, she would not go as far as calling Vladimir Putin a war criminal: "Ultimately it is for the international criminal court to make that determination but the evidence is there and New Zealand is supporting the prosecutors and making sure that Russia is held to account," Ardern said when pressed.
Asked why the Russian ambassador has not been expelled, she says New Zealand has been using "far more powerful tools" while keeping open those diplomatic options.
"When it comes to expelling ambassadors that also means you lose your diplomatic representation."
Ardern would not rule it out in future, but the other tools already being used have far more impact on the conflict, she said.
"It is an absolute option for us, it just so happens that we prioritised what we consider much more impactful options at this stage."
The Ministry of Foreign Affairs says eight New Zealanders in Russia, including individuals and families, have sought consular support or advice in the last month, with the Embassy in Moscow also receiving a small number of enquiries and requests for information.
There is more the government plans to do on sanctions, importation and looking at whether New Zealand can provide more support to help Ukrainian efforts on the ground, Ardern said.
Photo RNZ Caption: New Zealand Prime Minister Jacinda Ardern