Health authorities in Auckland are now testing a second school group for swine flu after it returned from Mexico on Saturday.
Ten students from Rangitoto College have tested positive for a strain of influenza likely to have begun in pigs after returning from Mexico, where as many as 81 have died from the virus.
The 22 students and three teachers on the trip are all confined to home.
Auckland Regional Public Health Service clinical director Julia Peters, confirmed on Monday that a group of 15 students from Northcote College is being treated in the same way
Dr Peters told Morning Report all the cases were being treated as swine flu until definitive tests come back from a Melbourne laboratory later in the week.
Drs & nurses meet incoming flights
Meanwhile, flights from North America coming into Auckland on Monday morning are being met by medical personnel.
The move is in response to an outbreak of a human form of swine flu that has killed up to 81 people in Mexico.
Swine flu - known as H1n1 - is a subset of influenza A.
The Government says 10 Rangitoto College students have influenza. They were part of a group of 22 students and three teachers who returned from Mexico on Saturday morning.
Radio New Zealand's health correspondent says they had coughs, fever, muscle aches and pains and blocked noses. Some also had diarrhoea. One student was admitted to North Shore Hospital.
The Government says it's likely the 10 students have swine flu, although this won't be known for several more days. Test samples are on their way to Melbourne.
One of the girls on the trip, who has tested negative, says they were on homestays for part of the trip and attended school with the Mexican students. They had been in Mexico for three weeks.
Rangitoto College is open as normal on Monday.
The Ministry of Health says anyone who has travelled to North America in the past couple of weeks and who has flu-like symptoms should get in contact with their doctor.
Health Minister Tony Ryall says four school parties are affected.
Those from Rangitoto College and Northcote College are already back in New Zealand.
Mr Ryall says two other school groups have been in Mexico, from Pinehurst and Westland Girls High School.
New Zealand is officially in 'Code Yellow'.
The Government is warning travellers returning from Mexico, Canada and Texas to see a doctor immediately if they develop any flu-like symptoms.
A notice posted on Sunday on the Government travel website advises travellers to take the same precautions as for any seasonal influenza including receiving a flu vaccine, washing their hands often, covering coughs and sneezes, and staying home when ill.
No contact yet
Meanwhile, health authorities have not yet contacted other passengers who flew back to New Zealand with students from Rangitoto College.
They all arrived on Air New Zealand flight NZ1 from Los Angeles at about 5am on Saturday.
Auckland Regional Public Health Service clinical director Julia Peters, told Morning Report her focus over the weekend was on managing the situation with the school group and her staff will begin following up other passengers on Monday.
But, she said, only those passengers who were sitting in seats around the school group will be contacted on Monday.
However, one of those sitting close to the students on the flight from Los Angeles, says he's concerned he has not yet been contacted.
Ian Hooker told Morning Report he does not know whether he should go to work on Monday.
Both he and his wife are teachers and Mr Hooker says they are worried about passing on the virus to their school.
Tamiflu reportedly effective
According to information being received from Mexico, Tamiflu is effective against swine flu. But there is no vaccine.
Radio New Zealand's health correspondent says New Zealand has large stockpiles of Tamiflu.
Biosecurity New Zealand says people are not at risk of catching swine flu from eating pork.
The agency says pigs are quite susceptible to the disease, but people can't catch it from consuming bacon or similar products.
It says the main way the disease can get into New Zealand is via incoming passengers.
Airline filter efficiency
The Air Line Pilots Association says Air New Zealand's air conditioning system is effective at filtering airborne viruses.
Air New Zealand claims the "high efficiency particulate air" filters in its air conditioning systems screen over 99% of viruses, including influenza.
While he cannot substantiate the airline's figures, Air Line Pilots Association technical director Hugh Faris says he understands - from discussions with its medical team - that the air conditioning system is very effective.
Meanwhile, the union representing international flight attendants plans to discuss the welfare of its members with Air New Zealand on Monday.
The Flight Attendants and Related Services Association says it is important to be proactive - rather than reactive - in these situations.