Ten positive influenza results from testing carried out on 13 students from Rangitoto College in Auckland who returned from Mexico on Saturday have been confirmed by Health Minister Tony Ryall.
Ministry of Health officials say there is no guarantee these students have the swine influenza, which is thought to have killed 81 people in Mexico, but they consider it likely.
All precautions are being taken to allow for this.
Mr Ryall said "I am advised 10 students have tested positive for Influenza A, and these results will now be sent to the World Health Organisation laboratory in Melbourne to ascertain whether it is the H1N1 swine influenza."
H1N1 influenza is a subset of influenza A.
None of the affected patients are considered seriously ill, and most are on the road to recovery.
An emergency decree has been declared in Mexico and the World Health Organisation has declared the outbreaks in Mexico and four US states as a "public health emergency of international concern".
Twenty-two students and three teachers from Rangitoto College were voluntarily being quarantined at home in Auckland on Sunday after returning from Mexico.
Mr Ryall says there are a total of four affected school parties.
He says two of them have already arrived back in the country from Rangitoto College and Northcote College.
Mr Ryall says he's told there are two more school groups to arrive back in New Zealand after having been in Mexico, from Pinehurst and Westland Girls' High School.
Mexico declares emergency decree
The World Health Organization has urged all countries to boost their surveillance for any unusual outbreaks of influenza-like illness and severe pneumonia.
Mexican President Felipe Calderon issued a decree on Saturday giving the government special powers to run tests on sick people, to isolate those who are ill, enter homes or workplaces and regulate air, sea and land transportation.
Of the 81 deaths suspected of being caused by swine influenza, 20 are confirmed. More than 1,300 people in Mexico were suspected to be infected with the flu.
All schools will be closed in Mexico City, the surrounding area and the central state of San Luis Potosi until 6 May. Cinemas and museums in Mexico City have been shut.
NZ authorities monitor situation
New Zealand's alert status for pandemic influenza has been upgraded from Code White to Code Yellow, one level down from Code Red, which would trigger a full scale response.
Mr Ryall said the Government is now taking every precaution.
The results from the Rangitoto College students will be sent to the World Health Organisation lab in Melbourne to establish if the disease is swine flu, with the results expected within the next few days.
Mr Ryall said some others who were on the tour party are also beginning to show symptoms.
He is calling for anyone who's been travelling the affected areas overseas and has flu-like symptoms to get in touch with their local health professional.
The Ministry of Health says it's set up a National Health Coordination Centre in Wellington to monitor developments.
The ministry's chief advisor for population health Greg Simmons says it's activated the early stages of its pandemic response plan.
Mr Simmons says pilots are always expected to be on the alert for flu-like symptoms in passengers, but that's especially so now.
He says customs officers have been asked to look for signs of respiratory illness in those returning from areas overseas where there is potential for this swine flu to be transmitted.
New Zealand's Ministry of Foreign Affairs and Trade has issued a travel health notice for swine flu in Mexico, California and Texas.
MFAT is advising New Zealanders who have recently travelled to any of the areas, and who develop flu-like symptoms, to seek immediate medical attention.
Auckland Airport says it's keeping a watching brief on the situation, though Air New Zealand does not fly directly into New York or Mexico.
New Zealand-based animal disease consultant, Professor Roger Morris, says this country has stockpiles of the medication Tamiflu which appears to work against the current strain of influenza.
Mr Ryall says stocks of Tamiflu have been released from Auckland's Middlemore Hospital to treat people with the symptoms.
Swine flu spreading
Probable or confirmed cases of the new strain of swine influenza have been found in New York, Kansas, California and Texas.
New York's health commissioner Dr Thomas Frieden said eight school students have been identified as probably having human swine influenza. About 100 students at the school in the borough of Queens have shown mild symptoms.
Further tests are being carried out to confirm that the virus is the deadly new strain of H1N1 swine flu that appeared in Mexico.
Kansas state health officials have also confirmed two cases, seven cases have emerged in California and three in Texas.
Meanwhile, in Britain, Northwick Park Hospital said a British Airways worker who arrived from Mexico City and fell ill does not have swine flu.
French health authorities have found two suspected cases of swine flu in travellers returning from Mexico and others are expected to follow, said the country's the top health official.