About 50 Japanese students kept in isolation at a Christchurch hotel with suspected swine flu will return home on Tuesday.
The Canterbury District Health Board says five of the girls have been tested and have been found to have the virus.
Canterbury medical officer of health Dr Alistair Humphrey says not everybody was tested so it cannot be assumed that only the five that are confirmed with swine flu have the illness.
"In fact the five that were swabbed have all come back positive so they've probably all got swine flu."
It's believed they carried the flu from Japan.
The students are among more than 200 15-year-olds on a cultural visit to New Zealand. The group is paying for the isolated students' hotel accommodation.
The others are being billeted in Hamilton, but the Canterbury DHB says they don't pose a risk as they're not showing symptoms. They are being treated with Tamiflu.
The Christchurch schools that the sick teenagers visited are on alert for signs of illness in their own students.
The visiting students are scheduled to fly back to Japan on Tuesday.
United States swine flu deaths surge
Meanwhile health experts in the United States have warned of shortages in swine flu vaccine supplies as deaths from the virus climb above epidemic levels in most states.
A medical official Anne Schuchat says of the 40 million doses expected by the end of October, up to 30 million might be available.
She says most of the country is experiencing an unprecedented increase in the number of deaths, especially among children, from swine flu in the space of a few weeks.