This article studies the realisability property of discrete-time bilinear and quadratic input-output (i/o) equations in the classical state-space form. Constraints on the parameters of the i/o model are suggested that lead to realisable models. Using new formulae for computing basis vectors of certain vector spaces of differential one-forms, we present in this article the complete list of the third-and fourth-order realisable i/o bilinear models, and a new realisable subclass of an arbitrary order is suggested. Moreover, we provide the sufficient conditions of the second-and third-order realisable i/o quadratic models, respectively. All the developed theory and algorithms are illustrated by means of several examples.