Despite being technologically very attractive, highly-doped zinc oxide whiskers with precisely defined morphology and doping level are difficult to prepare. Here, as an advancing step towards this goal, we show that pre-annealing of ZnO in oxygen-poor conditions (e.g. high vacuum) encourages a deeper diffusion of Ga into the ZnO crystal lattice in contrast to ZnO pre-annealed in oxygen-rich conditions. We also demonstrate that gallium acts as a reactant causing ZnO etching at diffusion temperatures, contrary to Al-based doping of ZnO systems. This behaviour, being similar to gallium melt-back etching during GaN epitaxy on silicon, has not been observed for ZnO so far and can represent a significant hurdle for the post-growth diffusion doping of ZnO nanostructures. The paper suggests possible ways how to diminish this effect.