Within the framework of density functional theory we have computed the total energy and the magnetic properties of wurtzite ZnO doped with diluted impurities of Co or of Ni in the presence of intrinsic vacancies. The complex MZn–VZn (M=Co,Ni) , where the Zn-vacancy (VZn) is in the next nearest neighbour site with respect the transition metal M atom, is energetically favored with respect to the configuration in which the two defects are far apart. Also for the more favourite thermodynamic growth condition the probability to find these types of complex at room temperature is relatively small (of the order of 10−7 or less).