The aim of this study was to determine the critical care and associated hospital costs for 2009 influenza A/H1N1 patients admitted to intensive care units (ICU) in Australia and New Zealand during the southern hemisphere winter All 762 patients admitted to ICUs in Australian and New Zealand between 1 June and 31 August 2009 with confirmed 2009 H1N1 influenza A were included. Costs were assigned based on ICU and hospital length-of-stay, using data from a single Australian ICU which estimated the daily cost of an ICU bed, along with published costs for a ward bed. Additional costs were assigned for allied health, overheads and extracorporeal membrane oxygenation services. The median (interquartile range) ICU and total hospital costs per patient were AU$35,942 ($10,269 to $82,152) and AU$51,294 ($22,849 to $110,340) respectively, while the mean (standard deviation) ICU and total hospital costs per patient were AU$63,298 ($78,722) and AU$85,395 ($147,457), respectively. A multivariate analysis found death was significantly associated with a reduction in the log of total costs, while the use of mechanical ventilation and ICU admission with viral pneumonitis/acute respiratory distress syndrome or secondary bacterial pneumonia were significantly associated with an increase in the log of total costs. The cost of 2009 H1N1 patients in ICU was significantly higher than the previously published costs for an average ICU admission, and the total cost of treating 2009 H1N1 patients in ICU admitted during winter 2009 was more than $65,000,000.