Almost a third of people infected with swine flu will show no symptoms but will still be able to spread the virus.
The Ministry of Health says up to half New Zealand's population could contract the A/H1N1 virus.
About 70 per cent of those will show symptoms. The rest could appear perfectly healthy and not know they are carrying - and spreading - the virus.
Christchurch virologist Dr Lance Jennings said the 30 per cent of infected people who had no symptoms still "shed" the virus, although they probably spread the disease less than those who developed symptoms such as coughing.
The ministry estimates that swine flu will kill up to 200 people in New Zealand.
The head of the ministry's national health co-ordination centre, Steve Brazier, told the health sector in an update of the pandemic action plan that the number of deaths was unlikely to exceed 200, "and more recent estimates suggest it may be substantially fewer".
Seasonal influenza usually kills more than 400 people a year directly or by exacerbating an underlying health condition.
"Children, young adults, people living in crowded conditions and people with chronic conditions have a greater risk of infection, but for most people [swine flu] is mild to moderate," Mr Brazier said.
The national tally of confirmed cases reached 303 yesterday, up 45 from Sunday.
But the ministry acknowledges that because public health workers are no longer testing all who might have the disease, the true number will be far higher.
No one has died in New Zealand from swine flu.
But an infected 30-year-old woman who is severely obese and has respiratory problems remained critically ill in Wellington Hospital yesterday.
Health authorities are moving to managing swine flu, rather than trying to contain it, although this is happening mainly in Auckland, Wellington and Christchurch, where the virus is spreading in the community.
Other areas will continue trying to contain the disease.
Dr Jennings said the effect of multiple infections - catching swine flu and regular flu - was unknown, but he expected people catching swine flu immediately after seasonal flu were likely to have worse symptoms because they would not have fully recovered from the first illness.
Under the shift to management, the ministry expects those with mild illness - which can still be debilitating - to be cared for at home, without medical assistance.
Only if a patient's symptoms worsen or if they have an underlying chronic health condition does the ministry want them to seek medical help.
Laboratory confirmation of swine flu and prescriptions of Tamiflu will also mostly be restricted to those with more-serious symptoms, underlying conditions that increase their susceptibility to complications, and institutional outbreaks.
State-funded vaccination against seasonal influenza has been extended by three months, until the end of September.
The Government wants to minimise the number affected by seasonal flu to reduce winter overcrowding of hospitals and free beds for swine flu patients.
The vaccination is available in primary health clinics at no charge to anyone aged 65 or older, and to those younger, including children who have any of a listed range of chronic conditions, including diabetes.
Others pay around $25 to $30, depending on the clinic's fees.
* What to do
Mild flu symptoms: Stay home, look after yourself, keep sick children home.
More-serious symptoms or underlying health problems: Phone your GP or Healthline (0800 611-116).
Source: Ministry of Health