  We have carried out observations of the newly-discovered magnetar in the
direction of Sagittarius A* using the Australia Telescope Compact Array in four
frequency bands from 4.5 to 20 GHz. Radio pulsations are clearly detected at
all frequencies. We measure the pulsar's dispersion measure to be 1650 +/- 50
pc/cm^3, the highest of any of the known pulsars. Once Faraday rotation has
been taken into account, the pulse profile is more than 80% linearly polarized
at all frequencies and has a small degree (5%) of circular polarization. The
rotation measure of -67000 +/- 500$ rad/m^2 is the largest (in magnitude) ever
measured for a pulsar but still a factor 8 smaller than Sgr A* itself. The
combination of the dispersion and rotation measures implies an integrated
magnetic field strength of -50uG along the line of sight. The flux density
appears to have increased by about a factor of two between observations made 30
days apart. This object therefore joins the small class of radio emitting
magnetars.
