Abstract Effluent and sludge samples from a number of dairy and piggery units in the North Island of New Zealand were collected and analysed for free ionic-copper (Cu 2+) and organically-complexed Cu. The bioavailability of sludge–Cu was examined using microbial respiration and plant growth experiments. Microbial respiration was measured at various levels of Cu (0–1000 mg kg −1), added as copper sulfate (CuSO 4) and sludge–Cu, using a Gilson differential respirometer. A glass house experiment was conducted to examine the transformation of Cu in soils and its subsequent uptake by ryegrass pasture. Three Cu sources were used that included fast-release CuSO 4, slow-release copper oxide (CuO) and Cu-enriched sludge. The pasture samples were analysed for Cu concentration. The transformation of Cu in the soil was monitored by analysing the soil samples for various fractions of Cu. The effluent and sludge samples collected from farms which regularly used Cu to treat lameness in dairy cattle and as a growth promoter in swine contained higher concentration of Cu. The total Cu concentration ranged from ∼0.1 to 1.55 mg l −1 and from 0.5 to 10.5 mg l −1 in the piggery and diary effluent, respectively. The corresponding values for the sludge samples were 3.0–526 and 25–105 mg kg −1. Most of the Cu in both the effluent and solid sludge material was organically complexed. The respiration measurements indicated that sludge–Cu was less toxic to soil microbial activity than CuSO 4. The results from the glass house experiment indicated that increasing the level of Cu applied through fertilisers and sludge increased Cu concentration in plants. At the same rate of application, plants took up less Cu from sludge and CuO than from CuSO 4. There was, however, a greater translocation of Cu from root to shoot at the highest rate of Cu through sludge application. The Cu fractionation study indicated that there was greater accumulation of organic bound Cu in the sludge-treated soil than the fertiliser-treated soil.