We investigated the efficiency of residual ozone from an advanced water treatment plant with an applied dose of 2.5 mg l−1 to inactivate viable Cryptosporidium oocysts during summer (i.e. 24°C) and winter (i.e. 18.9°C) in Queensland, Australia. Containers for sample collection were inoculated with 1,000 oocysts l−1 and filled with ozonated water. Ozone residual concentrations were measured at 0, 5 and 10 min intervals. Viability was determined by excystation. Non-ozonated water from the plant, trip and laboratory controls were also analysed. The applied ozone dose of 2.5 mg l−1 produced an immediate residual concentration of 1.25 mg 1−1 at 24°C and 1.34 mg 1−1 at 18.9°C in unseeded samples. The initial ozone residual in seeded containers was 1.22±0.03 mg 1−1 at 24°C and 1.37±0.04 mg 1−1 at 18.9°C. There was a gradual increase in inactivation of oocysts, with 49% of oocysts inactivated at 0 min to 92% after 10 min at 24°C and 57% at 0 min to 92.8% at 10 min at 18.9°C.