Rotorua District Council constructed a membrane bioreactor (MBR) system to work in parallel with the existing Bardenpho system, to improve the overall capacity and removal efficiency of the wastewater treatment plant. The aim of this research was to examine and compare the performance of the MBR and the Bardenpho systems, as well as the operating costs and the net present value. The MBR showed a higher removal rate of TKN, TSS, COD, DRP and organic nitrogen than the Bardenpho system, with average removal efficiencies of 94.52%, 99.05%, 94.04%, 67.43% and 92.75% respectively. These removal rates were shown to be 0.62%, 15.47%, 8.51%, 5.72% and14.74 % better than the Bardenpho system. Furthermore, the MBR removal rate remained stable over a range of flows, while the Bardenpho removal rate fluctuated with a change in flow. The total energy consumption of the treated wastewater in the MBR and the Bardenpho systems were 0.53 kWh/m3 and 0.25 kWh/m3 respectively. Major contributors to the total operational costs were the disposal cost of sludge, the cost of ethanol and aeration. In the MBR, the cost percentage of sludge disposal was 22.11%, and the total aeration cost was 20.25%, while the cost of disposal of sludge was 45.15% and aeration 7.22% in the Bardenpho. The capital cost to construct the MBR and the Bardenpho systems was calculated to be similar when the cost of land is not taken into account, however, the size of the Bardenpho required to treat the same volume of wastewater is found to be 2.2 times bigger than a MBR system. It was concluded that from a cost point of view, the Bardenpho system is more economical; however, the low cost of running the Bardenpho comes with lower removal efficiencies in comparison to the MBR.