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People who defy medical health officers' beefed-up swine-flu isolation powers risk being fined and forcibly detained.

The Government on Monday added swine flu to the list of notifiable diseases.

This allows people who have been in contact with patients who have swine flu to be compulsorily isolated at home, and pupils and teachers who are patients or contacts to be excluded from schools.

These measures are in addition to the general powers of medical officers of health under the Health Act 1956 to isolate people considered at risk of spreading swine flu.

A person who disobeys medical officers' orders faces a $500 fine, plus $50 for each further day of non-compliance.

But it is more likely the officer would invoke the power in section 79 (4) of the act to "detain him, by force if necessary".

Anyone who leaves their specified place of isolation is liable to be "arrested by any officer of the Ministry of Health or by any member of the staff of the hospital or other place of isolation or by any constable, without warrant" and taken to the place of isolation.

As well, medical officers of health gain much wider powers - such as restricting travel, closing businesses and banning meetings - if the Prime Minister issues an epidemic notice, the Minister of Health gives authority or a state of emergency is declared.

Then the penalty for disobeying a medical officer of health's directions rises to a maximum of six months' jail and a fine up to $4000.

The director of public health, Dr Mark Jacobs, said yesterday the Health Ministry's preference was voluntarily compliance and that was working well.

"If we had a problem with that, our fall-back wouldn't be to take them to court and fine them. It would be to enforce isolation, because the purpose of this is public health.

"Public health is not served by fining them."

The preference was to isolate people at home, or at hospital if necessary, he said.

Asked if police cells could be used, he said it was "unlikely but is in theory a possibility."

And he said it was unlikely the powers triggered by ministerial order or emergency declaration would be needed, even if swine flu - influenza, A (H1N1) - spread widely.

It has been contained in New Zealand; the tally of confirmed cased yesterday was 17.

Worldwide, more than 25,000 cases, including 139 deaths, have been reported. Australia's total has passed 1200 cases, mostly in Victoria, and is rising rapidly.

A group of Gisborne Boys' High school students who travelled home from a school trip to Brisbane on Sunday were yesterday isolated at school for the day.

Those with flu symptoms were given Tamiflu and asked to stay at home in isolation. The group includes 16 students, two teachers and a parent.

* The price of disobedience

$500 fine for refusing to be isolated.

$50 for every day of continued refusal.

Detention by force if necessary.