To assess the direct hospital costs for unplanned re-admissions within 30 days of hospitalisations with heart failure in Australia; to estimate the proportion of these costs attributable to potentially preventable re-admissions. Retrospective cohort study; analysis of linked admitted patient data collections data. People admitted to hospital (all public and most private hospitals in Australia) with primary diagnoses of heart failure, 1 January 2013 - 31 December 2017, who were discharged alive and re-admitted to hospital at least once (any cause) within 30 days of discharge. Estimated re-admission costs based on National Hospital Cost Data Collection, by unplanned re-admission category based on the primary re-admission diagnosis: potentially hospital-acquired condition; recurrence of heart failure; other diagnoses related to heart failure; all other diagnoses. The first two groups were deemed the most preventable. The 165 612 eligible hospitalisations of people with heart failure during 2013-2017 (mean age, 79 years [standard deviation, 12 years]; 85 964 men [51.9%]) incurred direct hospital costs of $1881.4 million (95% confidence interval [CI], $1872.5-1890.2 million), or $376.3 million per year (95% CI, $374.5-378.1 million per year) and $11 360 per patient (95% CI, $11 312-11 408 per patient). A total of 41 125 people (24.8%) experienced a total of 58 977 unplanned re-admissions within 30 days of discharge from index admissions; these re-admissions incurred direct hospital costs of $604.4 million (95% CI, $598.2-610.5 million), or 32% of total index admission costs; that is, $120.9 million per year (95% CI, $119.6-122.1 million per year), and $14 695 per patient (95% CI, $14 535-14 856 per patient). Re-admissions with potentially hospital-acquired conditions (21 641 re-admissions) accounted for 40.1% of unplanned re-admission costs, recurrence of heart failure (18 666 re-admissions) for 35.6% of re-admission costs. Unplanned re-admissions after hospitalisations with heart failure are expensive, incurring costs equivalent to 32% of those for the initial hospitalisations; a large proportion of these costs are associated with potentially preventable re-admissions. Reducing the number of unplanned re-admissions could improve outcomes for people with heart failure and reduce hospital costs.