• First characterization of magnetic dynamics of platelet suspensions using ACS. • Comparison of dynamics in isotropic and nematic phases. • Analysis of ACS spectra by using superposition of multiple Debye-type processes. • Investigation of influence of magneto- and electrostatic interactions. • Exploration of relationship between magnetic dynamics and macroscopic flow. Dense dispersions of ferrimagnetic bariumhexaferrite nanoplatelets form colloidal nematic phases even in the isotropic solvent such as n-butanol. The transition into the nematic phases is marked by developing the long-range orientational and magnetic correlations between the particles. Here we explore the magnetic dynamics in such dispersions in the range of the isotropic and nematic phases. We demonstrate that even in the low concentration isotropic regime, a simple Debye-type mechanism cannot describe the dynamics. Clustering of the particles results in low-frequency contributions.