  We investigate an obstruction for hypersymplectic manifolds equipped with a
free, isometric action of SU(1,1). When the obstruction vanishes, we show that
the manifold is a metric cone over a split 3-Sasakian manifold. Furthermore, if
the action of SU(1,1) is also proper, then the hypersymplectic manifold fibres
over a para-quaternionic Kahler manifold. We conclude the article with some
examples for which the obstruction vanishes. In particular, we show that the
moduli space to Nahm-Schmid equations admits a fibration over a
para-quaternionic Kahler manifold.
