Eleven cases of swine flu, mostly in older children and teenagers, have been confirmed in Otago and Southland.
Cases, all with mild symptoms, have been identified in Dunedin, Invercargill, Alexandra, Oamaru and Wanaka.
Medical officer of health Dr Derek Bell said there would be more unrecognised cases across the region.
Most people who had swine flu would not be identified as having the illness - "We're only testing a very small proportion of people who have flu".
Testing was being done in selected cases where it might make a difference to treatment, or where people had a severe illness or underlying conditions which placed them at greater risk, Dr Bell said.
He expected school absenteeism as a result of flu would spike when pupils returned to school after the fortnight's holidays, which begin today.
People mixed more during the holidays, increasing the opportunity for flu illnesses to spread.
People did not have to alter holiday plans, but sick people should be confined to home and measures to prevent the spread of illness, including scrupulous hand washing, should be followed.
