Pest monitoring methods should provide unbiased accurate estimates of pest densities and locations, while also minimizing time-in-field and costs. Recent pest mammal monitoring studies have found that chew cards are more effective than conventional mammal monitoring methods, but little experimental work has been done to determine optimal experimental duration or quantify the risks of saturation by one species biasing detections of other species. Here, we used chew cards in three sites within Awarua wetland (Southland, New Zealand) to investigate the optimal amount of time required to detect targeted pest species (rats, possums and mice), and to examine the potential of rats and possums to bias detection rates of other species. We found depressed detections of possums and rats where a contraspecific had been detected on a card, which is consistent with previous studies of a similar duration on interspecies interference. This experiment is the first to analyse the rates at which species detections accrue over the course of a survey, and we found rat detections lagged behind possums for the first four nights. We modelled the effect of survey duration and relative rat abundance on the likelihood of further possum detections. Duration and rat abundance interacted, meaning there are trade-offs to be considered with regard to duration: shorter durations may avoid the risk of saturation in areas of high pest density, but risk not sampling sparse or neophobic populations. Our data suggest that chew cards remain one of the most sensitive pest monitoring tools for rats and possums, compared to conventional methods such as tracking tunnels and wax tags. In areas of moderate pest densities, we suggest that a duration of five nights is optimal for detecting pests. However, in areas of high pest density the sensitivity of chew cards may render them unsuitable because of saturation and interspecies interference effects.