Context. Quantifying marine turtle-nest depredation by daily observer monitoring requires substantial labour.

Aims. To quantify nest depredation of the Vulnerable flatback turtle (Natator depressus) at one of its largest rookeries and to compare effectiveness of different monitoring methods.

Methods. We used daily observer monitoring and passive infrared-camera traps separately or in combination to record nest depredation, and identified impacts on remaining eggs and hatchlings in depredated nests.

Key results. More than a quarter (28%) of the 69 monitored nests were confirmed as depredated, although this figure is an underestimate of total losses because camera traps detected twice as many depredation events (39%) as did direct observation (17%) (P = 0.012). Cameras also provided important behavioural data and identified predators. Although reptile and bird predators were also recorded, the introduced red fox (Vulpes vulpes) was the primary predator identified, digging into 26% of the monitored nests. More than half of the depredation events by foxes (58% or 11/19 nests) occurred late term, between turtles hatching and emerging from the nest, which therefore means that losses calculated through counts of eggshell left in the nest are underestimates because predation of hatchlings is not included by the eggshell count method. Furthermore, almost half (42%) of all depredated nests were depredated more than once, with some nests opened up to five times, potentially exacerbating clutch losses due to environmental exposure.

Conclusions. Egg losses for confirmed depredated nests (27 ± 37%, range 0–100%) were three times the background levels (i.e. 9.3% of eggs that failed to hatch as a result of embryo death during development).

Implications. The results of this study strongly warrant the implementation of ongoing fox predator monitoring and mitigation strategies to protect nests at this nationally, and internationally, conservation-significant population of flatback turtles.