Vacancy-type defects in Mg-doped GaN were probed using a monoenergetic positron beam. GaN films with a thickness of 0.5-0.7 mu m were grown on GaN/sapphire templates using ammonia-based molecular beam epitaxy and characterized by measuring Doppler broadening spectra. Although no vacancies were detected in samples with a Mg concentration [Mg] below 7 x 10(19) cm(-3), vacancy-type defects were introduced starting at above [Mg] - 1 x 10(20) cm(-3). The major defect species was identified as a complex between Ga vacancy (V-Ga) and multiple nitrogen vacancies (V(N)s). The introduction of vacancy complexes was found to correlate with a decrease in the net acceptor concentration, suggesting that the defect introduction is closely related to the carrier compensation. We also investigated Mg-doped GaN layers grown using In as the surfactant. The formation of vacancy complexes was suppressed in the subsurface region (<= 80 nm). The observed depth distribution of defects was attributed to the thermal instability of the defects, which resulted in the introduction of vacancy complexes during the deposition process. Published by AIP Publishing.