The finding, published today in The Proceedings of the National Academy of Sciences, suggests these new, sometimes untreatable forms could become widespread across the globe.

A research team led by University of NSW senior lecturer Mark Tanaka used mathematical methods to examine properties of different strains of Mycobacterium tuberculosis.

Dr Tanaka said scientists had believed resistant bacteria paid a price for their ability to defy antibiotics in that they spread among humans more slowly than other strains. But the researchers found this was not always the case.

''Overall, they're transmitting just as much as the drug-sensitive strains. That's the call for concern,'' Dr Tanaka said.

''They're not just evolving and then dying off as lineages, they're succeeding as well.''

One in three people carry the tuberculosis bacterium, though it remains latent in most cases. Only people who are sick with the bacteria in their lungs are infectious and, like a common cold, it spreads through the air when they cough, sneeze, talk or spit.

The World Health Organisation estimated there were 9.27 million new cases of tuberculosis in 2007 worldwide and 1.6 million deaths related to the disease in 2005. Australia sees about 1000 new cases a year.

Dr Tanaka said developed countries had almost overcome tuberculosis, but it was a major problem throughout developing nations.

''It's one of these diseases that we think of as something we controlled a century ago, but actually it's not true at all,'' he said.

Canberra Hospital infectious diseases and microbiology director, Professor Peter Collignon, said the study highlighted the importance of stopping the spread of not only tuberculosis but also other drug-resistant bacteria, such as golden staph.

''Other issues, like nutrition and better housing and social issues, are really important too, because that all means your ability to fight the infection is better, plus you're less likely to spread it,'' Professor Collignon said.