The Kremlin, which denies being behind the attack, responded by saying it would expel British diplomats from Moscow.
Speaking at Polyfest in Auckland today, Ardern denied that New Zealand had been soft on Russia over the attack.
"We have labelled this action as repugnant. It is a complete breach of international law. We've already used our voice in The Hague to reiterate that. Of course, we stand firmly against it."
Earlier this week Ardern again re-emphasised that a free trade deal with the European Union was New Zealand's top priority, following an interview with Foreign Minister Winston Peters in which he talked favourably about Russia.
Peters had said on The Nation that there was "no evidence" Russia had a hand in shooting down flight MH17 over the Ukraine or that it had meddled in the US election.
He added that New Zealand traded with countries that had, for example, human rights or gender equality issues.
National Party foreign affairs spokesman Todd McClay said Peters was "fast earning himself an international reputation as an apologist for Russia".
"His inexplicable push for a free-trade agreement is raising questions and concerns among our closest partners – and threatening our own trade agenda. The Prime Minister needs to shut him down and start showing some leadership to our friends and allies."
Late last year European Union ambassador Bernard Savage said that any thawing of relations between New Zealand and Russia would be viewed in a "very negative" light.
Meanwhile, Ardern has issued a strongly worded statement saying new Zealand supported the position of the leaders of the UK, US, Germany and France on the nerve-agent attack in Salisbury.
"Outrage at the brazen and callous use of chemical weapons in a UK town is fully justified. This attack left three people seriously injured, including a police officer who assisted at the scene, and potentially threatened many more," she said.
"This incident is a serious affront to accepted global rules and norms. The use of chemical weapons in any circumstances is totally repugnant, and New Zealand is deeply disturbed at any use of chemical substances banned by the Chemical Weapons Convention.
"New Zealand fully supports the sovereign right of the UK to take the action it considers appropriate in response to this violation of international law on its territory. We stand in solidarity with the UK alongside its other partners.
"Despite the further details that have emerged since the NZ government statement earlier this week, and despite the international outcry, the Russian reaction has been cynical, sarcastic and inadequate.
"There is no plausible alternative explanation hitherto, that this came from anywhere other than Russia, and no doubt whatsoever that Russia has serious questions to answer."
