Since 2006, alum (aluminium sulphate; Al₂(SO₄)₃) has been applied to the Utuhina and Puarenga Streams at a targeted dose rate of 1 mg Al L⁻¹ to control phosphorus loading to Lake Rotorua. Alum dosing is widely used for water quality restoration and, under circumneutral pH (6-8), is considered to have no significant toxicological impacts at low to moderate (0.1–2 mg Al L⁻¹) concentrations. At circumneutral pH, alum forms aluminium hydroxide (AlOH₃), a white insoluble precipitate which adsorbs phosphorus, reducing its availability for phytoplankton growth. However, at high or low pH, monomeric and hydroxy aluminium species occur in varying proportions with respect to acidic (i.e., Al³⁺, AlOH²⁺, Al(OH)²⁺) and alkaline (i.e., Al(OH)₄⁻) conditions with increasing toxicity to aquatic organisms. Under typical conditions, the pH of Lake Rotorua is near pH 7, but due to its limited buffering capacity may reach pH 10 during intensive algal blooms. These diel increases in pH occur due to the photosynthetic uptake of CO₂ during the day, thereby increasing the environmental hydroxide concentration and raising the pH of the lake. At night, respiration releases CO₂, driving down the pH due to the formation of carbonic acid. Diel pH cycling has the potential to solubilise alum-derived aluminium, resulting in toxicological impacts on aquatic biota. Previous research has primarily focused on the toxicological impacts of aluminium under acidic or, more recently, alkaline conditions, however, potential impacts during transient exposure to alkaline pH have not been reported. The University of Waikato was contracted to investigate the effects of aluminium at 2 mg L⁻¹ in association with diel pH cycling on rainbow trout (Oncorhynchus mykiss), common bully (Gobiomorphus cotidianus) and kōura (Paranephrops planifrons) osmoregulation and respiration. Potential osmoregulatory effects were investigated by exposure of rainbow trout and kōura to aluminium at 2 mg L⁻¹ under diel pH cycling (pH 7–10) over 10 days. No significant differences between control and treatment groups were observed in either plasma and haemolymph osmolarity, haematocrit or haemoglobin concentration (Student’s t-test, P >0.05). A significant difference (Student’s t-test, P <0.05) in the mean cell haemoglobin concentration (MCHC) in the rainbow trout control group was attributed to erythrocyte swelling, suggesting a generalised stress-induced response rather than an impact from aluminium. Histological examination of kōura and rainbow trout gill tissue was also conducted, with abnormalities observed within both control and treatment groups for each species. A significant difference (Kolomogorov-Smirnov, P <0.05) was observed between kōura control and treatment groups, indicating that kōura gills may be more susceptible to erosional damage from precipitated aluminium hydroxide. It was concluded that diel pH cycling and exposure to 2 mg Al L⁻¹ were unlikely to significantly impact osmoregulatory function in rainbow trout and kōura in Lake Rotorua. A second experiment utilised intermittent flow respirometry to determine mass-specific metabolic oxygen consumption rates (MO₂) of rainbow trout, common bully, and kōura exposed to 2 mg Al L⁻¹ and diel pH fluctuations (pH 7–10) over 60 hours. It was expected that precipitation of dissolved aluminium onto the gill surface during pH transitions and/or binding of dissolved aluminium species could inhibit the respiratory gas exchange resulting in increased gill ventilation from hypoxic and hypercapnic conditions. There was a significant negative correlation (Pearson’s, r² = 0.34, P <0.001) between pH and MO₂, possibly indicating kōura were sensitive to the experimental conditions. However, there were no consistent trends in differences between control and treatment groups for any of the tested species. Data interpretation was hindered by the necessity to exclude samples from the rainbow trout and common bully control groups due to aberrations in the oxygen sensor data. However, the mean MO₂ was within expected ranges for rainbow trout and kōura, indicating that significant respiratory impacts were not occurring during the exposure period. Although definitive conclusions could not be made regarding changes to MO₂ in response to aluminium and diel pH cycling, it is unlikely that acute impacts would result from these conditions in the natural environment. From this research, it is concluded that the combined impacts of diel pH cycling and aluminium exposure at twice the current dosing rate (i.e., 1 mg Al L⁻¹) are unlikely to significantly impact respiratory and osmoregulatory function in fish and kōura in Lake Rotorua. There was some indication that at circumneutral pH and moderate aluminium concentrations (2 mg Al L⁻¹), particulate aluminium may cause erosional damage to kōura gill tissue. However, such conditions are highly unlikely to be encountered within the lake where the mean total aluminium concentrations are approximately one hundred times lower (0.02 mg Al L⁻¹). These experimental results are consistent with observations from the Al-dosed Utuhina stream, which show no evidence of effects on macroinvertebrate communities or kōura downstream of the Al input. Further investigation of aluminium exposure on more susceptible and less mobile life stages (e.g., larval rainbow trout) would help to define potential impacts and species tolerances. Further testing of potential impacts on gill tissues of kōura from particulate aluminium is also recommended.