An expert in disease control said New Zealand cannot stop the spread of a new strain of swine flu simply by stopping travel, New Zealand media reported on Sunday.

Roger Morris, an international disease control consultant, said at this stage the virus seems to react to Tamiflu and any early cases which arrived New Zealand would be treated with the drug, the Newstalk ZB radio reported on Sunday.

An emergency decree has been declared in Mexico following the outbreak of a new flu virus suspected of killing 81 people there.

The World Health Organization has declared the outbreaks of influenza-like illness in Mexico and four U.S. states as a "public health emergency of international concern".

The flu spread from person to person, rather than being caught from pigs.

But health officials have stopped short of declaring a pandemic.

Morris said stopping travel movement around the world will not stop the virus spreading.

He added that there will be enough people incubating the disease who will get through the system and continue to spread it.