  We give a general formula for generators of the NL-cone, the cone of
effective linear combinations of irreducible components of Noether-Lefschetz
divisors, on an orthogonal modular variety. We then fully describe the NL-cone
and its extremal rays in the cases of moduli spaces of polarized K3 surfaces
and hyperk\"ahler manifolds of known deformation type for low degree
polarizations. Moreover, we exhibit explicit divisors in the boundary of
NL-cones for polarizations of arbitrarily large degrees. Additionally, we study
the NL-positivity of the canonical class for these modular varieties. As a
consequence, we obtain uniruledness results for moduli spaces of primitively
polarized hyperk\"ahler manifolds of ${\rm{OG6}}$ and ${\rm{Kum}}_n$-type.
Finally, we show that any family of polarized hyperk\"ahler fourfolds of
${\rm{Kum}}_2$-type with polarization of degree $2$ and divisibility $2$ over a
projective base is isotrivial.
