In this paper, a numerical implementation of a eddy viscosity-type wave breaking model in open source HOS-NWT code (Ducrozet et al. 2012) is presented along lines similar to Seiffert & Ducrozet (2018). Due to Eulerian formulations of free surface boundaries in High Order Spectral (HOS) method, wave breaking can’t be simulated directly. Hence, detection of onset of wave breaking followed by an appropriate dissipation mechanism needs to be modelled. For this purpose, the aforementioned model is deployed in HOS-NWT. This is followed by a subsequent validation against breaking wave groups generated by second-order corrected focusing signals in intermediate depths under laboratory conditions (Sriram et al. 2015). The mechanism of dissipation is studied against a time-frequency analysis with particular focus on high frequencies that seem to dominate the wave breaking regime. © 2019 Taylor & Francis Group, London.