Increasing numbers of cases of swine flu are being detected that are resistant to Tamiflu, the drug the UK and rest of the world stockpiled to fight a pandemic, according to scientists calling for greater global monitoring.

Even more worryingly, these strains of flu are appearing in patients who have never been treated with the drug, which means the strains are able to pass from one human to another.

Tamiflu, generic name oseltamivir, is one of the few treatments available for pandemic swine flu, although it is thought to be of limited effectiveness. The reluctance of the manufacturer Roche to release all the trial data has made it difficult to ascertain how limited. Nonetheless the drug can save lives if used early in the course of the illness.

Resistance to the drug has been shown before, but the new Australian data on its steady growth and the apparently easy transfer from one person to another of Tamiflu-resistant flu strains will alarm public health experts.

The data comes from Dr Aeron Hurt, of the World Health Organisation collaborating centre for reference and research on influenza in Melbourne. He and his colleagues have found that approximately 2% of H1N1pdm09/swine flu cases are resistant to Tamiflu. While that is not large, an increasing proportion of the patients have never been treated with the drug – so they have contracted a form of flu that is already resistant to it.

In 2011, the team detected the most widespread cluster to date of Tamiflu-resistant H1N1 swine flu cases in Newcastle, New South Wales. This, they say, is further evidence of the ability of this drug-resistant strain to circulate in the community.

Tamiflu-resistant strains of H1N1pdm09 have been identified in Europe but only on an ad hoc basis, said Hurt. "However, the trend observed in Australia of a greater proportion of resistant cases being detected in untreated community patients is also being observed both in the USA and Europe," he said.

Hurt, who presented his findings to the annual scientific meeting of the Australasian Society for Infectious Diseases in Canberra, said animal studies had confirmed that the latest drug-resistant strains could be passed on more easily than drug-resistant strains in the past.

"Sustained global monitoring for the emergence of resistance is important to underpin public health and guidance for clinical management. Surveillance schemes should assess frequency of resistance in the community and in specific patient groups receiving treatment, such as severely immunocompromised, seriously ill patients in hospital, and patients not responding to antiviral therapy," said Hurt.

"Further studies to better understand influenza-virus infections in these patients and to improve antiviral treatment strategies are needed."