Russia's ambassador to New Zealand says Russia "has nothing to do" with the use of the military-grade nerve agent known Novichok in an attack against an ex-spy in the UK.

Valery Tereshchenko told RNZ's Morning Report his country denied involvement with the attempted murder of Sergei Skripal, 66, and his daughter, Yulia Skripal, 33.

Both remain in a critical condition in hospital after being found on March 4.

"I'm sure this event is a well-staged provocation aimed at fanning anti-Russian campaign," Tereshchenko said.

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"It is quite strange as far as the Skripal matter is concerned that the experts defined the type of toxin very fast and they quickly found an antidote."

If that antidote was not used the Skripals would not have survived, he said.

British Prime Minister Theresa May ordered the expulsion of 23 Russian diplomats believed to be involved in espionage-related activities in the wake of the attack.

She gave them a week to leave the country.

"Russia has nothing to do with what happened to Mr Skripal and his daughter, Yulia," Tereshchenko said.

It was "surprising that the British side has made conclusions so fast".

"People who can think logically might ask why does Russia need that right now?"

Russia signed the Chemical Weapons Convention (CWC) and carried out its obligations by eliminating all chemical weapons, he said.

"According to the information that was spreading in the country the USSR used to allegedly produce such agents but that was done in the laboratory that was situated in Uzbekistan.

"I can not say for sure where it was brought from, if it really was that toxic agent Novichok that somehow turned out in territory of the United Kingdom..."

He added that a chemical specialist who allegedly developed that agent had fled from Russia in 1986.

British High Commissioner to New Zealand Laura Clarke told Morning Report it was "very clear" the Novichok used in the attack had stemmed from Russia.

"Our view is either that means it was a direct act by the Russian state, or the Russian Government has lost control of its stock of Novichok, this military-grade nerve agent," she said.

The Russian Government "despite our invitation" had not provided a plausible explanation, she said.

The attack fitted into "a pattern of Russian aggressive behaviour" including the illegal annexation of Crimea, the violation of European airspace and hostile cyber attacks, she said.