World Health Organisation director-general Dr Margaret Chan warned yesterday that Zika posed a threat of "alarming proportions", with new cases reported in 23 countries in South America and elsewhere.

"It is now spreading explosively," she said. "The level of alarm is extremely high."

Dr Jose Derraik, a senior research fellow at the University of Auckland's Liggins Institute, said the risk of an outbreak in New Zealand was low compared to other countries because the mosquitoes which carried the virus were not present in this country.

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Zika virus: What you need to know


"However, we do not know whether the mosquitoes present in New Zealand can transmit [the] Zika virus," he said.

The Ministry for Primary Industries said yesterday that its biosecurity systems at the border were already robust and it was not scaling up its work in response to the Zika situation.

The spokesman said an exotic mosquito was most likely to arrive on a passenger aircraft or in pools of water on board a ship. For this reason, all aircraft arriving in New Zealand are sprayed with insecticide and vessels are routinely checked for pools of water.

The Ministry of Health's chief medical officer Dr Don Mackie said in most cases, the Zika virus was benign.

However, it has recently been linked to birth defects in Brazil, where 4000 cases of infection have been confirmed.

It has also been spreading more rapidly than usual.

Dr Derraik said this was probably because Latin American populations had never been exposed to the virus and therefore had no immunity to it, meaning it was able to spread more quickly.

The Ministry of Health said eight of the New Zealanders who had contracted the virus had recovered but one of them, a 47-year-old man, was admitted to Waikato Hospital.

The man, who had returned from Tonga on January 15, had symptoms of Guillain-Barre, a disease linked to Zika which causes paralysis. He was in a stable condition and was expected to make a full recovery.

Dr Mackie said the man was likely to be in some discomfort but not feeling any sharp pain. The paralysis could take weeks to subside, he said.

Of the nine New Zealanders who had contracted the virus this year, four had been in Samoa, four in Tonga, and the other person's travel destinations were not known.

The ministry was advising pregnant women not to travel to Samoa and Tonga.

New Zealand athletes who planned to compete at the Olympic Games in Rio de Janeiro in August have been told by New Zealand's Olympic Committee not to travel there if they are pregnant or planning to get pregnant in the near future.

Some airlines are offering refunds for people who have booked fares to affected countries.