Biodegradable coatings and films of cellulose nanofibers (CNFs) or a combination of CNFs and inorganic fillers, such as clay or calcium carbonate (CaCO3), can provide a replacement for non-biodegradable plastic coatings as barrier layers in packaging boards. In this work, transparent composite films were prepared from CNFs of Pinus radiata and Eucalyptus using different amounts of clay and CaCO3 as fillers. The impact of raw material (softwood vs. hardwood), TEMPO oxidation levels and filler type (clay vs. CaCO3) on film properties was studied. Pinus radiata CNF films had superior mechanical properties to Eucalyptus CNF films, but no significant differences were observed in the barrier and optical properties. Clay seemed to work better as filler compared to CaCO3, in terms of its impact on film properties. Composite films with CaCO3 as filler were highly brittle with inferior properties to clay-CNF films, and an uneven distribution and agglomeration of the CaCO3 mineral particles was evident in SEM images. Based on the results, clay as filler in CNF coatings is preferred for targeting packaging board applications. Rheological characterisation of the CNF suspensions revealed shear-thinning behaviour, with the CNF from Eucalyptus having higher viscosities and lower power-law indices when compared to the CNF from P. radiata.