The US has announced it is expelling 60 diplomats believed to be intelligence officers and many EU countries have also expelled diplomats. It comes after the UK's initial move to boot out 23 diplomats – a move which was matched by Russia.
Further pressed on the issue this morning, Ardern said she reserved the right to expel any Russian undeclared intelligence officers that come to light - but the NZ Security Intelligence Service had assured her that there were none in New Zealand.
"I wouldn't want you to draw in between the lines of my comments here. What I'm very cautiously saying at this point is that those who fit the bill of those who are being expelled by other countries, as far as we're aware, are not present here in New Zealand."
She said it did not defy belief that Russia had no undeclared intelligence officers in New Zealand.
"When you take into account Australia has found, if I recall correctly, two, then perhaps it's not that difficult to believe that we don't have anyone in New Zealand that fits that description.
"If we have anyone that fitted the description of the likes of what Australia and our partners have expelled, we would be expelling them too."
She defined an undeclared intelligence officer as "someone who works in the intelligence space, but doesn't want to overtly declare their interest in that area".
She said the Ministry of Foreign Affairs and Trade had spoken directly to the Russian Ambassador "to express our concerns, which we've made very public".
Russia has described the expulsions as "provocative" and indicated it will again retaliate in kind.
Ardern has come under fire for what was seen as an initially slow response by herself and Foreign Minister Winston Peters to British Prime Minister Theresa May's call for international partners to join the UK in condemning Russia.
Ardern said New Zealand had done the same as other partners, including making strong statements about Russia's failure to tender an explanation for the presence of its nerve agent in the UK and calling for a direct response.
The Skripals were poisoned in the southern England city of Salisbury on March 4. Testing revealed the Russian-made nerve agent Novichok has been used. Russia has refused to concede it was responsible.
