  Adjusting conventional Chern-Simons theory to ${\rm G}_2$-manifolds, one
describes ${\rm G}_2$-instantons on bundles over a certain class of
$7$-dimensional flat tori which fiber non-trivially over $T^4$, by a pullback
argument. Moreover, if $c_2\neq0$, any (generic) deformation of the ${\rm
G}_2$-structure away from such a fibred structure causes all instantons to
vanish. A brief investigation in the general context of (conformally
compatible) associative fibrations $f:Y^7\to X^4$ relates ${\rm
G}_2$-instantons on pullback bundles $f^*E\to Y$ and self-dual connections on
the bundle $E\to X$ over the base, a fact which may be of independent interest.
