We study the moduli space of \(G_2\) -instantons on (projectively) flat bundles over torsion-free \(G_2\) -orbifolds. We prove that the moduli space is compact and smooth at the irreducible locus after adding small and generic holonomy perturbations. Consequently, we define the \(G_2\) -Casson invariant that is invariant under \(C^0\) -deformation of torsion-free \(G_2\) -structures. We compute this invariant for some orbifolds that arise in Joyce’s construction of compact \(G_2\) -manifolds.