In the mountains of the Gold Coast Hinterland (Queensland, Australia), Euastacus sulcatus (Decapoda: Parastacidae) is the largest aquatic invertebrate, largest member of the aquatic shredder guild, and probably plays a key role in the area’s stream ecology. The objective of this study was to investigate the ecological effects of E. sulcatus in the streams of the Gold Coast Hinterland, but in particular to assess the species’ capacity to act as an effective ecosystem engineer, obtain measures of the ecosystem services provided by the species, and evaluate the likely impacts of the crayfish on downstream water quality. To achieve this objective, an estimate of the population size of E. sulcatus was obtained; a series of experiments measured the species’ capacity to liberate nutrients from leaf litter, reduce the particle-size of the leaf litter, and mobilise and redistribute organic and inorganic materials in a flowing stream environment. This information allowed estimation of the species’ ecological impact(s) per unit time as a measure of the ecosystem services provided by E. sulcatus in the headwater streams of the study area. During this study E. sulcatus was common and abundant with a within-stream density of ~0.5 crayfish m-2. The total population of E. sulcatus in the study area was estimated at >860,000 crayfish. Correction for strong seasonal variation in activity was required for ecological impact calculations, and the “ecologically effective” population was estimated at >570,000 crayfish (biomass ~12 tonnes wet weight (WWT)). Crayfish significantly liberated nitrogen and phosphorus from leaf litter into the water column via consuming leaf litter and excreting nutrients. The phosphorus dynamic was complex, however the overall effect of crayfish was to facilitate eventual “loss” of phosphorus from the system with binding/absorption to substrates, particularly leaf litter, evident as the mechanism. The ecologically effective population was estimated to liberate at least 140 kg of nitrogen, 1.5 kg of phosphorus, and facilitate the loss of around 0.15 kg of soluble phosphorus from leaf litter per annum. Crayfish consumed large amounts of leaf litter while significantly reducing its particle-size. The ecologically effective population consumed at least 14 tonnes WWT of whole leaf litter, producing ~4.5 tonnes WWT of fine particles (<1 mm), with a nett consumption of ~9 tonnes WWT leaf litter per annum. The day-to-day activities of crayfish led to significant mobilisation and downstream export of materials 1 mm diameter, with about half this material of organic origin. The ecologically effective population facilitated the mobilisation and downstream export of around 90 tonne-meters (dry weight) material per annum. Given this annual crayfish-export effect is dramatically overwhelmed by the periodic flood spates in the area, it is unlikely crayfish-mediated export of materials is negative for downstream water quality. In the field it is anticipated that the liberated nutrients will be stripped from the water column by other biota, and/or abiotic processes, and eventually recycled back into and retained in the headwater ecosystems. Similarly, crayfish-mediated particle-size reduction of leaf litter will facilitate further processing by other biota, and recycling and retention of this material in the headwater regions. In the typical baseflow conditions, the instream mobilisation of materials will be positive for maintaining the high quality of water flowing from the headwater streams, by maintaining streambed complexity and ecological processes such as enhanced decomposition of organic detritus. Euastacus sulcatus is a common, abundant and ecologically aggressive species, and the results of this study support a conclusion that this species is an effective ecosystem engineer. The population of E. sulcatus clearly provides a number of ecosystem services, and even lower bound estimates indicate the magnitude of these services are fairly substantial at a local scale, and must be of appreciable benefit in maintaining the function and health of the headwater stream ecosystems.