  We present the high angular resolution catalogue for the Australia Telescope
20 GHz (AT20G) survey, using the high angular resolution 6-km antenna data at
the baselines of ~ 4500 m of the Australia Telescope Compact Array (ATCA). We
have used the data to produce the visibility catalogue that separates the
compact Active Galactic Nuclei (AGNs) from the extended radio sources at the
0.15 arcsec angular scale, corresponding to the linear size scale of 1 kpc at
redshifts higher than 0.7. We find the radio population at 20 GHz to be
dominated by compact AGNs constituting 77% of the total sources in the AT20G.
We introduce the visibility-spectra diagnostic plot, produced using the AT20G
cross-matches with lower frequency radio surveys at 1 GHz (the NRAO-VLA Sky
Survey (NVSS) and the Sydney University Molonglo Sky Survey (SUMSS)), that
separates the 20 GHz population into distinct sub-populations of the compact
AGNs, the compact steep-spectrum sources, the extended AGN-powered sources and
extended flat-spectrum sources. The extended flat-spectrum sources include a
local thermal emitting population of high latitude planetary nebulae and also
gravitational lens and binary black hole candidates among the AGNs. We find a
smooth transition in properties between the compact CSS sources and the AGN
populations.
  The visibility catalogue, together with the main AT20G survey, provides an
estimate of angular size scales for sources in the AT20G and an estimate of the
flux arising from central cores of extended radio sources.
  The identification of the compact AGNs in the AT20G survey provides high
quality calibrators for high frequency radio telescope arrays and VLBI
observations.
