Cellulose nanofibres (CNF) derived from spinifex grass were surface modified via grafting with an epoxidized soybean oil (ESO), which improved its hydrophobicity. The method was successfully applied to CNF-derived nanopaper, as well as a paper containing micro- and nanofibres to compare substrate morphology effects. The water absorption capacity of ESO-grafted nanopaper was reduced by 40% compared to untreated nanopaper, and the contact angle of cellulose nanopaper grafted with ESO reached 99°. The chemical surface analyses (FTIR and XPS) revealed that a relatively thin, less than 10 nm layer of ESO-polymer was covalently bound to the surface. The method was also successfully applied to a freeze-dried CNF aerogel, which showed a reduction of water vapour absorption of up to 53%. Characterisation using solid state NMR confirmed the covalent attachment of ESO to the CNF aerogel. Our work provides an essential premise in expanding the functionality and potential use of different forms of CNF in diverse applications requiring higher hydrophobicity.