  We present the results of an Australia Telescope Compact Array (ATCA) survey
for intra-day variability (IDV) of the total and polarized flux densities of
118 compact, flat spectrum, extragalactic radio sources from the Parkes 2.7 GHz
Survey. A total of 22 total flux density IDV sources were discovered and 15
sources were found to show IDV of their polarized flux density. We discuss the
statistical properties of the IDV sources, including the distribution of source
modulation indices, and the dependence of the variability amplitude on source
spectral index and on Galactic position. We suggest interstellar scintillation
(ISS) in the Galactic interstellar medium as the most likely mechanism for IDV.
Even so, the inferred high brightness temperatures cannot be easily explained.
