Cetacean entanglements in fishing gear cost governments, fishermen and stakeholders millions of dollars a year, and often result in serious injury or death of the entangled animals. Entanglements have been implicated in preventing the recovery of some large whale populations. Acoustic deterrents on fishing nets are widely used to reduce incidental captures of dolphins and porpoises, but there is little evidence as to whether they effectively deter large whales. We tested whether a low-frequency whale alarm (3 kHz Whale Pinger®, 135 ± 5 db, 5 s emission interval and 400 ms emission duration) deterred Southern Hemisphere humpback whales Megaptera novaeangliae from approaching a potential source of entanglement. Northerly migrating humpback whale pods were tracked by an observer blind to alarm status (on/off) as they passed an alarm moored in the centre of the peak migration path. Of 137 pods tracked, 82 (60%) passed within the assumed detectable range (500 m) of the alarm, 51/78 (65%) when it was on and 31/59 (52%) when it was off(p = 0.18). There was no discernible response to the alarm. Whale pods did not differ in directionality, course heading or dive duration when within detectable range of the alarm, whether it was on or off, and a number of pods passed directly over the alarm while it was operational. This suggests that single alarms as currently configured and attached to a trap or pot line are unlikely to effectively deter humpback whales from approaching potential hazards, at least during their northerly migration phase.