The number of confirmed cases of influenza A/H1N1 in New Zealand rose to 109 on Tuesday, a sign that the flu epidemic was escalating rapidly.

The Ministry of Health said 23 new cases of the H1N1 virus had been confirmed in the previous 24 hours, 13 from South Island's Canterbury region, seven from Auckland and three from Wellington. There are other 21 probable cases.

Three schools and two early childhood centers were now closed as the flu continued to spread through the community.

Students at three Christchurch and four Auckland schools have come down with the virus.

The Ministry of Education said on Tuesday there were at least three other schools which have cases of the virus but are remaining open.

The Ministry of Education said other schools which have cases of flu are instituted partial closures with groups of students in the hundreds told to stay at home.

Health Minister Tony Ryall said the Ministry of Health was stepping up advertising mainly around the "helping not to spread the flu" message.

Authorities were trying to delay the inevitable further increase in influenza A/H1N1 cases.

The aim was to delay the peak beyond the normal winter seasonal increase in flu cases, Ryall said.

"Normally we would see about 31,000 medical admissions to hospitals a month during winter. Officials think we could end up with another 4,000 or so if the swine flu peak matches the winter peak," he said.

The Ministry of Health was warning against people deliberately contracting influenza A/H1N1, as widespread absences from work and school would have a severe negative impact on the community.

Media reports in Australia and the United States said some parents have intentionally exposed their children to influenza A/ H1N1 flu, believing that infecting them now will save them from a potentially more virulent strain in the future.

Deputy director of public health Fran McGrath said whether getting the virus to gain immunity to it was a good strategy would be known in the future, but at this stage New Zealand had a relatively small number of cases and the ministry wanted that to continue through the normal flu season.

"For individuals it tends to be a mild to moderate illness-- it can be serious for some," she said. "But it's the impact of widespread absences, business continuity, schools and workplaces, that is like to be severe impact."

McGrath said despite the increasing numbers, there are still relatively few cases and only isolated instances of the virus being transmitted in the community.

About 1,500 calls were made to Healthline on Monday, which is more than double the usual number.

Surveillance by Environmental Science and Research, which runs the National Influenza Center in Upper Hutt, showed that in the first four weeks of the normal flu season, some regions are showing high weekly consultation rates for flu-like illness at medical practices.

The Ministry of Health has moved its response to phase 6.2 which means restrictions could be placed on public gatherings and a state of emergency could be declared. It is the last phase before code red.

Health officials say many cases of influenza A/H1N1 are likely to go unreported as the virus continues to spread.

Wellington's Medical Officer of Health Margot McLean said many people who come down with the virus will go undiagnosed and it will become increasingly difficult for testing to tell the difference between influenza A/H1N1 and seasonal influenza.