Acoustic deterrent devices are frequently used as a mitigation method to exclude harbour porpoises Phocoena phocoena from areas of potential harm, such as wind farm construction sites. However, there is increasing evidence that the devices themselves have the capacity to cause hearing damage. Here, we investigated the response of harbour porpoises to a 15 min sequence of 200 ms sound (peak frequency 10.5 kHz, range 5.5-20.5 kHz, 27 sounds total), which elicits the acoustic startle reflex. We used a duty cycle (0.6%) and sound exposure level that were significantly lower than in conventional acoustic deterrent devices. Harbour porpoises were exposed to startle sounds from a small vessel, and groups were visually tracked during 13 sound exposure sequences and 11 no-sound control trials. Porpoises showed a significant avoidance reaction during exposure, travelling a mean distance of 1.78 km (max. 3.21 km). In all cases, they left the area within 1 km of the sound source in the first 15 min after the start of the startle sequence. No avoidance was exhibited during control trials. Results are consistent with the startle reflex mediating this behaviour at low response thresholds. Our method can be used for mitigating collision risk and the risk of hearing damage from renewable energy installations, their construction and the deterrence device itself.