A self-consistent linear scaling three-dimensional fragment (LS3DF) method is used to study the dipole moments and internal electric fields of large ZnO nanorods. Our ab initio calculations reveal that the ZnO nanorod with unpassivated (1010) side surface has a side surface contribution per Zn-O dimer 10 times larger than the bulk contribution per Zn-O bond. Detailed analysis is used to decompose the total dipole moment into different contributions. It is found that the total dipole moment of a nanorod can be calculated from the different parts with their dielectric screening described by a continuous model. We also show the effect of the dipole moment on the interior electronic structure of the nanorod.