This study examined the postprandial metabolism and swimming speed of southern bluefin tuna Thunnus maccoyii when fed sardines Sardinops sagax of either high-lipid and high-energy content or low-lipid and low-energy content. Five groups of two or three T. maccoyii (mean ±s.e. mass = 19·8 ± 0·5 kg, n = 14) were fed either low [2·2% lipid, 5·5 MJ kg−1 gross energy (GE)] or high-lipid (12·9%, 9·2 MJ kg−1 GE) S. sagax. Before feeding, T. maccoyii swam at 0·74 ± 0·03 body lengths s−1 (n = 5) and their routine metabolic rate was 305 ± 15 mg kg−1 h−1. Swimming speed and metabolic rate of T. maccoyii increased following feeding. Thunnus maccoyii swam 1·3 and 1·8 times faster during digestion of low and high-lipid S. sagax, respectively. Postprandial peak metabolic rate, duration of elevated metabolism and total postprandial metabolic increment were all greater for T. maccoyii that ingested high-lipid S. sagax. When total postprandial increment is represented as a proportion of ingested energy, there was no difference between high and low-lipid meals, equating to between 30 and 35% of ingested energy. It was estimated that increased postprandial swimming costs account for 25 and 46% of the total postprandial metabolic response for low and high-lipid S. sagax meals, respectively. Specific dynamic action (SDA) accounts for c. 20% of ingested energy regardless of S. sagax lipid level. These results confirm that the postprandial metabolic increment of T. maccoyii is greater than most other fish species. Much of the high cost of postprandial metabolic increment can be attributed to increased postprandial swimming costs. For T. maccoyii, it appears that activity and SDA are not independent, which complicates bioenergetic evaluation. High postprandial metabolic costs accentuate the great energetic requirements of T. maccoyii.