Precise identification and quantification of radioisotope in a sample largely depend on the accuracy of the full-energy peak efficiency of the detector. It is generally observed that the efficiency for a given energy of point-like sources is not the same as the extended source. However, number of correction factors such as detector geometry, photon attenuation, coincidence-summing, etc. may reduce such a difference in efficiency regardless of sources, or they should be considered in the measurement of extended samples. In connection to this, the variation of absolute photopeak efficiency of a high-purity germanium (HPGe) detector with the distances from the surface of the detector as well as with the γ-ray energies were investigated using $\gamma$-ray standard point sources. We present a method to determine the loss of efficiencies in radioactivity measurement of the thin extended radioactive disc samples in comparison to point source.