We have shown that epidemic curves and estimates for the effective reproduction number confirm the very different nature of the pH1N1 outbreaks in Victoria and WA. There appeared to be no pandemic virus in WA prior to multiple importations from Victoria, other Australian jurisdictions and overseas. Estimates of R in WA were consistently around 1.2 and never above 1.4. By contrast, initial estimates of R in Victoria approached 3, followed by a rapid -and implausible -decrease over a very short period of time. This precipitous fall cannot be explained satisfactorily by potential reduced transmission associated with early control interventions (such as partial school closure, t isolation and treatment of cases, and chemoprophylaxis and home quarantine of contacts), or depletion of susceptible people within the population. For instance, it took 20 days for the estimated value of R for the 1918-19 pandemic in San Francisco to fall from ,2.4 to ,1.2 [9] . We suggest the high initial value of R = 2.7 in the Victorian pandemic and the dramatic decline in 9 days is more likely explained by unrecognised cases due to an earlier commencement of the epidemic in Victoria.