Understanding how fish respond to changes in their environment provides us with essential information on their needs, facilitating improved management. We examined the abiotic drivers of variability in activity of Murray cod (Maccullochella peelii), a large freshwater fish endemic to south-eastern Australia. Accelerometer-derived activity was continuously recorded for six months from 18 individuals occupying a discrete reach of river. Activity was highest during crepuscular and nocturnal periods when water temperatures were warmest (19–30°C; January – March). As water temperatures cooled (9–21°C; April – June), Murray cod were active throughout the full diel cycle and dormant periods were rarely observed. Light level, water temperature and river discharge all had a significant, non-linear effect on activity. Activity peaked during low light levels, at water temperatures of ~20°C, and at moderate discharge rates within in the normal operating range for this system. The temporal changes observed in the behaviour of Murray cod likely reflect the complex interactions between physiological requirements and prey resource behaviour and availability in driving activity.