Non-orthogonal multiple access (NOMA) has emerged as a promising technology for 5G networks and beyond. In order to fully reap the benefits of NOMA, it is essential to characterize its performance under different channel fading. In this paper, we carry out a performance analysis of the downlink NOMA-based dual-hop mixed powerline/wireless communication (PLC/WLC) system. Specifically, we investigate the system reliability and spectrum efficiency in terms of the outage probability and the ergodic capacity, respectively, where the PLC link undergoes lognormal fading while the WLC link undergoes block Rayleigh fading. The accuracy of the analytical results has been validated, the effect of different system controlling parameters is tested; and the optimal values of power allocation factors are obtained. Also, the system performance is compared to a benchmark system through representative extensive simulations.