Elevated levels of nutrients and suspended sediment (SS), and changes to other environmental parameters, are frequently associated with forestry harvesting (clearfelling) operations, and are indicative of the potentially complex changing environment associated with clearfelling. Current and future recommended best management practices (BMPs) for forestry clearfelling on upland peat catchments must provide for a healthy soil and good water quality. The aim of this study was to quantify the effects of implementation, or violation, of BMPs in the clearfelling of an upland peat conifer forest. Over periods of 12 months prior to clearfelling and 15 months after clearfelling, two peatland forests, comprising a study control (no clearfelling) and a study site (clearfelling), were monitored for the release of phosphorus (P) and nitrogen (N) species (dissolved reactive phosphorus (DRP), total phosphorus (TP), total oxidised nitrogen (TON) and ammonium nitrogen (NH4+-N)), SS, dissolved oxygen (DO), electrical conductivity (EC), pH and stream water temperature. Clearfelling was conducted during poor weather conditions and a watercourse, which drained the study site, was not protected. The maximum recorded concentration exported from the study site after clearfelling was 471 µg L-1 for DRP, 611 µg L-1 for TP, 1336 µg L-1 for NH4+-N, and 194 µg L-1 for TON. Concentrations of SS exiting the study site increased in one of the two samples taken during clearfelling (maximum release of 481 mg L-1, with 68% of this organic) and returned to pre-clearfelling levels, or below, within 6 months of the commencement of clearfelling. Exports of TP and DRP from the study site were 0.9 and 0.4 kg ha-1 yr-1, which were greater than the study control (0.6 and 0.2 kg ha-1 yr-1, respectively). This indicated that the mitigation practices employed on site were not effective in phosphorus retention.