Only four examples of complete non-compact $G_2$ manifolds are currently known. In joint work with Mark Haskins and Johannes Nordstrom we construct infinitely many families of new complete non-compact $G_2$ holonomy manifolds. The underlying smooth 7-manifolds are all circle bundles over asymptotically conical (AC) Calabi-Yau manifolds of complex dimension 3. The metrics are circle-invariant and their geometry at infinity is that of a circle bundle over a Calabi-Yau cone with fibres of fixed finite length. The $G_2$ manifolds we construct are therefore 7-dimensional analogues of 4-dimensional ALF hyperKaehler metrics. The dimensional reduction of the equations for $G_2$ holonomy in the presence of a Killing field was considered by Apostolov-Salamon and by several groups of physicists. We reinterpret the dimensionally-reduced equations in terms of a pair consisting of an SU(3) structure on the 6-dimensional orbit space coupled to an abelian Calabi-Yau monopole on this 6-manifold. We solve this coupled system of non-linear PDEs by considering the adiabatic limit in which the circle fibres of the associated circle-invariant $G_2$ holonomy metrics collapse. The $G_2$ holonomy metrics we construct should be thought of as arising from abelian Hermitian-Yang-Mills connections on AC Calabi-Yau 3-folds, especially AC Calabi-Yau metrics on crepant resolutions of Calabi-Yau cones.