Two days after that event, with Russia facing significant resistance in Ukraine, the Russian President announced the mobilisation of 300,000 reservists, the first such move by the country since World War II.

With separatist leaders in Russian-occupied Ukrainian provinces announcing referenda to join Russia, Putin also said he would use "all available means" to fight back if Russian territory was threatened. It led to overwhelming criticism from world leaders, including Prime Minister Jacinda Ardern.

Speaking from Parliament on Thursday, Mahuta said the proposed referenda were "absolutely concerning" and she "would be really worried about next steps".

"Any threat to use nuclear weapons will harm many people beyond the shores of Ukraine and we can't accept it," Mahuta said, describing Putin's announcement as "an increased escalation of tension".

She said New Zealand was continuing to assess how to respond to the situation and said our response so far has been "credible alongside our international partners".

Since the Russian invasion in February, the Government has defended not expelling the Russian Ambassador to New Zealand by saying his presence here allows diplomatic channels to remain open. Kicking him out would also risk Moscow reacting by expelling our own representatives there.  

Mahuta on Thursday made it clear the option remains on the table, saying she will "leave open the question of whether or not to expel the Russian Ambassador".

"In this circumstance, I leave open the prospect that we may have to consider the status of the Russian Ambassador here in New Zealand," said Mahuta. "It will be an active consideration".

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"The position of New Zealand is to keep open diplomatic channels in order to de-escalate the situation. But again, we assess our position in relation to the actions of Putin. Right now, hosting a referendum that would impact on the sovereignty of Ukraine appears to be an escalation rather than a de-escalation of the situation."

After initially saying expelling diplomatic staff "is not currently an option", MFAT later confirmed it "does remain an option". 

"If we expel the Russian Ambassador, we could anticipate a reciprocal action to our Embassy in Moscow." 

Sarah Walsh is New Zealand's Ambassador to Russia. A career diplomat, Walsh started in the role in January, but only this week presented her credentials to the Russian President.

The Russian Embassy in New Zealand on Wednesday posted a picture of Walsh waiting with 23 other foreign ambassadors at an event in Moscow attended by Putin.

At the ceremony, Putin had a message for New Zealand. 

"Let me remind you that during World War II, New Zealand, as part of the coalition of allies, fought together with the Soviet Union against Nazism, against this misanthropic ideology. It is well known that the current state of affairs in bilateral relations cannot be considered satisfactory."

Newshub asked MFAT why Walsh was there considering Russia's actions towards Ukraine and New Zealand's condemnation of them.  

"Presentation of credentials, typically to the Head of State, is a step under the Vienna Convention on Diplomatic Relations and ensures an ambassador is able to fully perform their functions and role," a spokesperson said.

"Ambassador Walsh did not have any direct contact with President Putin during the ceremony, but in parallel to the Russian credentials ceremony, the Aotearoa New Zealand Government took the step of reiterating our position regarding Russia's illegal, unprovoked and unjustified attack in a formal communication to the Russian Government."

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Mahuta said the New Zealand Ambassador "provides opportunities for consulate support for those Kiwis who are in Russia or maybe in the areas that she's accredited to". New Zealand currently advises New Zealanders "do not travel" to either Russia or Ukraine. 

World leaders, many of whom are currently in New York for the United Nations General Assembly, have severely criticised Putin's nuclear threats and said his actions are evidence the Russian military is failing to take Ukraine

Ardern said threatening to use nuclear weapons on Ukrainian territory was hypocritical considering Putin framed the Russian invasion as a quest to liberate those in the region. She said the announced referenda were a "sham". 

"What we need here is a rallying cry from the world. What is happening in Ukraine is illegal, it's immoral, it's causing the loss of civilian lives and that loss could extend if, as Putin has claimed, he broadens the type of weapons he uses in this war."

She met with Ukrainian Prime Minister Denys Shmyhal on Thursday morning (NZT), during which they discussed the latest developments. 

"PM Ardern expressed New Zealand's ongoing support for Ukraine and reiterated strong condemnation of Russia's illegal war including escalation in Putin's recent speech and the sham referendums," a statement said. 

PM Shymhal thanked Ardern for NZ's strong, swift and ongoing support. They discussed future work around post-conflict reconstruction and the work through the legal institutions to hold Russia to account for their actions in the war."

In its statement, the MFAT spokesperson said any attempts by Russia to annex the occupied regions of Luhansk, Donetsk, Zaporizhzhia and Kherson would be a "further act of aggression" in breach of Ukrainian sovereignty and territorial integrity.

"Aotearoa New Zealand does not recognise any attempt to change Ukraine's borders, and we continue to convey our support for Ukraine's sovereignty and territorial integrity.

"Russia's illegal and unprovoked invasion of Ukraine has already caused thousands of deaths, a massive humanitarian crisis and untold suffering. 

"Any steps by Russia that risk a further escalation of the war in Ukraine, are reckless and irresponsible. As we have from the start of Russia's illegal, unprovoked invasion, we continue to call on President Putin to act consistently with international obligations, cease Russia's invasion of Ukraine, withdraw troops from Ukrainian territory and return to diplomatic negotiations as a pathway to resolution of the conflict."