A levelling network was readjusted and a new geoid model compiled within the framework of geodetic vertical datum modernization at the Hong Kong territories. To accomplish all project objectives, the quasigeoid model has to be determinedtoo. A quasigeoid model can be obtained from existing geoid model by applying the geoid-to-quasigeoid separation. Thegeoid-to-quasigeoid separation was traditionally computed as a function of the simple planar Bouguer gravity anomaly, whiledisregarding terrain geometry, topographic density variations, and vertical gravity changes due to mass density heterogeneities below the geoid surface. We applied this approximate method because orthometric heights of levelling benchmarksin Hong Kong were determined only approximately according to Helmert’s theory of orthometric heights. Considering afurther improvement of the accuracy of orthometric heights by applying advanced numerical procedures, we determinedthe geoid-to-quasigeoid separation by applying an accurate method. The comparison of the accurately and approximatelycomputed values of the geoid-to-quasigeoid separation revealed signifcant diferences between them. The approximatevalues are all negative and reach -2.8 cm, whereas values from the accurate method vary between -4.1 and+0.2 cm. In addition, we assessed the efect of anomalous topographic density on the geoid-to-quasigeoid separation by employing a newlydeveloped digital rock density model. According to our estimates the efect of anomalous topographic density reaches amaximum value of 1.6 cm, refecting a predominant presence of light volcanic rocks and sedimentary deposits at the HongKong territories. Our numerical fndings indicate that the conversion between geoid and quasigeoid models should be doneaccurately, even in regions with a moderately elevated topography