The electronic structure of nickel dichalcogenides, NiS2 and NiSe2, in monolayer form, is studied employing first-principles methods. We assess the importance of band ordering, covalency, and Coulomb interactions in the ground state of these systems. Hybrid functional results are compared with standard functionals and also with Hubbard-corrected functionals to systematically address the role of electronic interactions and localization. Using empirical as well as linear response derived and modified parameters, we found that mean-field correlations realized by intersite Hubbard interactions are directly linked to the magnitude of the energy band gap. This gives compelling evidence that the DFT+U+V method is suitable to describe the charge transfer insulating phase in these materials.