The behavior of threading dislocations (TDs) in ZnO/(0001)sapphire heterostructures grown by a two-step method have been investigated. The initial template, grown by pulsed laser deposition, consisted of a continuous underlayer, which was O-polar and an overlayer comprising a high density of Zn-polar nanorods. High densities (∼7×1010 cm−2) of TDs were found to be restricted to the underlayer, whereas the nanorods were almost defect-free. Subsequent treatments by either hydrothermal growth or chemical vapour deposition (CVD) achieved epitaxial lateral overgrowth of nanorods and led to continuous Zn-polar films. The low TD density of nanorods remained until misoriented grain boundaries and boundary dislocations were generated when neighbouring nanorods become coalesced. The lateral migration of TDs in the overgrowth led to dislocation interactions and reduction of TDs. The total TD density at the top of the overlayer was estimated to be ∼1×109 cm−2 for hydrothermal growth and ∼7×109 cm−2 for CVD growth.