Zinc oxide (ZnO) nanoparticles were synthesised by a mechanochemical method using ZnCl2 and Na2CO3 as precursors and NaCl as a diluent. The effects of the milling energy, influenced by factors such as the milling time and ball-to-powder mass ratio, and the ordering of the synthetic stages on the structure, morphology and optical properties of the synthesised nanoparticles were characterised by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy, as well as by UV-visible and photoluminescence spectroscopies. The results indicated the direct formation of ZnO nanoparticles after 10 hours of milling, without the need for heat treatment, as a result of the high-energy milling process. The crystallite sizes of the nanoparticles increased with higher milling times and ball-to powder mass ratios due to cold welding. ZnO nanoparticles with crystallite sizes in the range of 16 nm–19 nm were produced through optimisation of the order of synthetic stages and milling energy. Different morphologies were obtained in different samples due to the effects of changing the order of the synthetic steps. The optical band gap of the synthesised ZnO nanoparticles was lower than in bulk samples because of the presence of defects.