As the death toll in Mexico from swine flu cases climbs to more than 100, New Zealand health authorities are preparing for a pandemic.

Today suspected swine flu cases were being monitored nationwide, including Auckland, Hamilton, Palmerston North, Nelson, the West Coast, Canterbury and Otago and some were being treated with Tamiflu as a precaution.

They included 10 people from Auckland's Rangitoto College, who have tested positive for influenza A, from the same family as swine flu, and three more from Northcote College whose were awaiting test results.

Health officials believe it is likely they have swine flu, a deadly new strain of influenza A, which Mexican Health Minister Jose Angel Cordova says has now claimed 103 lives in Mexico.

Samples from the students who tested positive for influenza A are being sent to the World Health Organisation laboratory in Melbourne to ascertain whether it is the subset H1N1 swine influenza. The results should be back in a number of days.

Prime Minister John Key said that so far only a few people who had travelled to affected areas had shown symptoms, and they had been mild.

"The only thing we have identified so far is a form of influenza," he said.

"Swine flu, which is H1N1, is yet to be confirmed."

Health Minister Tony Ryall said two thirds of the passengers on Air New Zealand flight NZ1 which arrived from Los Angeles with the Rangitoto College passengers on board had been contacted or had presented themselves to health professionals.

The Ministry of Health would soon start contacting passengers on NZ5, the flight which brought the Northcote College students home.

Both groups had been in Mexico.

In brief:
* Swine flu is a subset of the influenza A virus known as H1N1
* The virus has killed 103 people in Mexico and infected over 1600
* Twenty cases have been confirmed in the US, six in Canada
* There are suspected cases in France, New Zealand, Spain, Israel and Brazil
* Northcote and Rangitoto College groups returned from Mexico on Saturday
* Ten Rangitoto students have tested positive for influenza A and the group is being kept in home isolation
* Three Northcote College students and two Waikato residents are also being tested for influenza A
* Passengers arriving in NZ on North American flights are being screened and given information on the flu virus
* The potential influenza epidemic status has been upgraded to yellow

Air New Zealand group general manager international airline Ed Sims said that as a precautionary measure, the 18 crew and two staff travelling for work purposes on NZ1 on Saturday had all been told to stay at home for up to seven days in accordance with ministry guidelines.

The Health Ministry said it was important that people who had travelled to Mexico or North America in the past fortnight should contact Healthline (0800 611 116) for information. They should seek medical advice if they displayed flu-like symptoms.

Royal New Zealand College of General Practitioners president Jonathan Fox said today patients with flu-like symptoms should not turn up to GPs' waiting rooms.

"There was immediate risk the virus would spread," he said.

"When you ring the general practice, tell the receptionist you have flu-like symptoms and want to speak to your GP or the person who is taking such calls."

Earlier it was revealed the Rangitoto College group at the centre of the swine flu alert thought they might be having a bad reaction to Mexican food.

A Rangitoto school teacher stayed in North Shore Hospital overnight due to the symptoms of the disease, but she was expected to be discharged later today. A student was admitted on Saturday but has since been discharged.

All the infected students and their families have been quarantined at their homes.

The other students and teachers on the trip stayed home today as a precaution, while five students who played soccer with one of the returning students - who had no symptoms - were also sent home today as a precaution.

Click here for the Ministry of Health's influenza pandemic action plan.

Rangitoto College principal David Hodge said most of those believed to have the virus were responding well to doses of the anti-influenza drug, Tamiflu.

"I became aware on Saturday that the kids had arrived home safely but aware that there had been illness on the trip," he told NZPA.

"Originally it was put down to Mexican food, which seemed like a good guess at the time."

The group were Spanish language students having three weeks in Mexico to give them language and cultural experience.

Global spread

Canada today became the third country to confirm human cases of swine flu with public health officials reporting six people falling ill in Nova Scotia and British Columbia.

In the US, health officials said 20 swine flu cases had been reported so far in New York, Ohio, Kansas, Texas and California. There are also suspected cases in France, Israel, Brazil and Spain.

On Saturday, the World Health Organisation activated stage three of its six-stage global influenza plan. Stage three puts nations on "pandemic alert", urging them to intensify surveillance for unusual outbreaks of influenza-like illness and severe pneumonia, but is less serious than the declaration a pandemic is under way.