  We describe radio continuum and spectral line observations of the source
G320.4-01.2 (MSH 15-52) and the coincident young pulsar B1509-58 made with the
Australia Telescope Compact Array. Based on a comparison between X-ray and
radio observations, we argue that the two main radio components of G320.4-01.2
are a single supernova remnant (SNR), which HI absorption indicates is at a
distance of 5.2+/-1.4 kpc. A high-resolution correspondence between radio and
X-rays argues that the pulsar is interacting with the SNR via an opposed pair
of collimated outflows. The outflow itself is seen as an elongated X-ray
feature surrounded by a highly polarized radio sheath, while the interaction
with the SNR manifests itself as a ring of radio/X-ray knots within the optical
nebula RCW 89. We reject the hypothesis that the pulsar outflow powers the
entire RCW 89 region.
  SNR G320.4-01.2 and PSR B1509-58 agree in distance and in rotation measure,
and appear to be interacting. We thus conclude that the two objects are
associated and have an age of <~1700 yr. We propose that the SNR resulted from
a high-energy or low-mass supernova which occurred near the edge of an
elongated cavity. Such a model can account for the SNR's bilateral appearance,
its large apparent age, the significant offset of the pulsar from the SNR's
centre and the faintness of the pulsar-powered nebula at radio wavelengths.
