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OBJECTIVE: To chart emergency department (ED) attendance and acute admission following a devastating earthquake in 2011 which lead to Canterbury's rapidly accelerated integrated health system transformations. DESIGN: Interrupted time series analysis, modelling using Bayesian change-point methods, of ED attendance and acute admission rates over the 2008-2014 period. SETTING: ED department within the Canterbury District Health Board; with comparison to two other district health boards unaffected by the earthquake within New Zealand. PARTICIPANTS: Canterbury's health system services ∼500 000 people, with around 85 000 ED attendances and 37 000 acute admissions per annum. MAIN OUTCOME MEASURES: De-seasoned standardised population ED attendance and acute admission rates overall, and stratified by age and sex, compared before and after the earthquake. RESULTS: Analyses revealed five global patterns: (1) postearthquake, there was a sudden and persisting decrease in the proportion of the population attending the ED; (2) the growth rate of ED attendances per head of population did not change between the pre-earthquake and postearthquake periods; (3) postearthquake, there was a sudden and persisting decrease in the proportion of the population admitted to hospital; (4) the growth rate of hospital admissions per head of the population declined between pre-earthquake and postearthquake periods and (5) the most dramatic reduction in hospital admissions growth after the earthquake occurred among those aged 65+ years. Extrapolating from the projected and fitted deseasoned rates for December 2014, ∼676 (16.8%) of 4035 projected hospital admissions were avoided. CONCLUSIONS: While both necessarily and opportunistically accelerated, Canterbury's integrated health systems transformations have resulted in a dramatic and sustained reduction in ED attendances and acute hospital admissions. This natural intervention experiment, triggered by an earthquake, demonstrated that integrated health systems with high quality out-of-hospital care models are likely to successfully curb growth in acute hospital demand, nationally and internationally.