Activated carbon has found key applications in the adsorption of polluted industrial dyes in water. In this work, macadamia husk biochar (MHC) was prepared using a household pyrolysis kiln before being activated with phosphoric acid (H3PO4) to obtain macadamia-husk-activated carbon (MHAC). A preliminary study was made on two activation conditions of MHC:phosphoric acid (w/v), 1:1 and 1:3, for the removal of malachite green (MG) dye. Analysis of experimental results revealed that the adsorption process was highly controlled by the time of contact, MHAC particle size, MHAC dosage and initial dye concentration. With the use of an MHAC particle size of 125–202 μm, an MHAC dosage of 6 g/l and a contact time of 120 min, the removal efficiency reached >99% at an MG concentration of 40 parts per million (ppm) before being degraded to around 75% at 70–80 ppm MG. Impregnation with zinc nitrate hexahydrate (Zn(NO3)2·6H2O) on the MHAC surface could maintain a removal efficiency of >99% at all initial dye concentrations (40–80 ppm), so the maximum removal capacity increased to ∼130 mg/g.