The thermodynamic conditions of the Zn vapour in the growth atmosphere found in carbothermal reduction vapour phase transport growth of ZnO nanostructures are discussed. In typical growth conditions the Zn vapour is undersaturated and will only nucleate at energetically suitable accommodation sites such as Au-coated regions or ZnO buffer layers/seeds. We suggest that growth proceeds by Zn condensation at such sites and subsequent oxidation, consistent with experimental evidence that metal catalysts or pre-deposited ZnO nucleation points are required for ZnO nanostructure growth using carbothermal vapour phase transport.