Intermittently Decanted Extended Aeration (IDEA) processes are widely used for wastewater treatment. However, in-depth performance evaluation of a full-scale IDEA plant is rare, making it challenging for water utilities to meet the increasingly stringent discharge requirements with these assets. This study aims to fill this gap through a comprehensive assessment of nitrogen and phosphorus removal in a full-scale IDEA plant in Australia. The plant consists of two identical IDEA tanks operated in-parallel. Upstream to each tank is a bioselector with four interlinked compartments. We conducted an eight-week monitoring program with four intensive cyclic studies to establish detailed nutrient profiles of the two IDEA tanks to assess the performance of nitrogen and alum assisted phosphorus removal. Results showed that the plant enabled good nitrification in the IDEA effluent. However, the denitrification efficiency was low (ca. 50%), and could be improved by decreasing oxygen supply to suppress nitrite oxidation and preserve influent carbon. The addition of alum to the IDEA tank appeared to be ineffective given the low P concentration (<1 mg-P/L) in the tank. The bioselector was identified as a better alum-dosing location, given its higher (~7-fold) phosphate concentration in comparison to the influent. Stopping the dosing of alum only marginally increased the effluent P (0.35 to 0.52 mg-P/L), implying that P removal was predominantly (94%) biologically mediated and achieved via P accumulating microorganisms. Overall, this study offers timely and useful process understanding of the performance of IDEA plants, as well as other similar wastewater treatment configurations.