The ionic conduction in a novel solid sodium-ion conductor of Na2Zn2TeO6 (NZTO) is investigated from the point of view of defect chemistry. NZTO shows an ionic conductivity of 0.57 mS cm−1 at room temperature, and the grain bulk conductivity and the grain-boundary conductivity are individually measured using the AC impedance spectroscopy at temperatures down to −30 °C. The grain-boundary conductivities are about two orders of magnitude lower than those of the grain bulk; such a phenomenon can be ascribed to the Schottky barrier at the grain boundaries of the NZTO electrolyte. The concentration and mobility of the charge carriers in the grain bulk are calculated from the grain bulk conductivity. The concentration and mobility of the charge carriers and the Schottky barrier height can be tuned by doping; the ionic conductivity of NZTO is enhanced to 0.83 mS cm−1 by the doping of 2.5 mol% Ga at the Zn sites, because the Ga-doping increases the concentration and mobility of the charge carriers, and lowers the Schottky barrier height.