We theoretically investigate mixtures of nematic liquid crystals (NLCs) and graphene nanoplatelets, focusing on their phase and structural behaviour. The NLC are characterized using the mesoscopic nematic tensor order parameter, which defines the local amplitude of orientational order and its spatial orientation. Our analysis begins by determining conditions necessary to prevent phase separation. For this purpose, we introduce the effective Flory-Huggins parameter, which reveals how NLC order and platelet surface treatment influence the phase separation. Then we illustrate the impact of NLC order on the orientational ordering of immersed graphene. Finally, we qualitatively examine distinct effects of the immersed graphene on the NLC phase behaviour, which may exhibit either critical or subcritical behaviour. This study highlights the complex interplay between NLC order and graphene nanoparticles, offering insights into how these interactions influence overall system properties. A detailed understanding of the mutual interaction between NLC and graphene nanoplatelets could enable the development of innovative applications based on combined extraordinary properties of both materials.