We present the full source catalogue from the Australia Telescope 20 GHz ( AT20G ) Survey .
The AT20G is a blind radio survey carried out at 20 GHz with the Australia Telescope Compact Array ( ATCA ) from 2004 to 2008 , and covers the whole sky south of declination 0° .
The AT20G source catalogue presented here is an order of magnitude larger than any previous catalogue of high-frequency radio sources , and includes 5890 sources above a 20 GHz flux-density limit of 40 mJy .
All AT20G sources have total intensity and polarisation measured at 20 GHz , and most sources south of declination -15 \degr also have near-simultaneous flux-density measurements at 5 and 8 GHz .
A total of 1559 sources were detected in polarised total intensity at one or more of the three frequencies .

The completeness of the AT20G source catalogue is 91 per cent above 100 mJy beam ^ { -1 } and 79 per cent above 50 mJy beam ^ { -1 } in regions south of declination -15 \degr .
North of -15 \degr , some observations of sources between 14 - 20 hr in right ascension were lost due to bad weather and could not be repeated , so the catalogue completeness is lower in this region .
Each detected source was visually inspected as part of our quality control process , and so the reliability of the final catalogue is essentially 100 per cent .

We detect a small but significant population of non-thermal sources that are either undetected or have only weak detections in low-frequency catalogues .
We introduce the term Ultra-Inverted Spectrum ( UIS ) to describe these radio sources , which have a spectral index \alpha ( 5 , 20 ) > +0.7 and which constitute roughly 1.2 per cent of the AT20G sample .

The 20 GHz flux densities measured for the strongest AT20G sources are in excellent agreement with the WMAP 5-year source catalogue of , and we find that the WMAP source catalogue is close to complete for sources stronger than 1.5 Jy at 23 GHz .