The Planck Australia Telescope Compact Array (Planck-ATCA) Co-eval Observations (PACO) have provided flux density measurements of well-defined samples of Australia Telescope 20GHz (AT20G) radio sources at frequencies below and overlapping with Planck frequency bands, almost simultaneously with Planck observations. We have observed with the ATCA a total of 482 sources in the frequency range between 4.5 and 40GHz in the period between 2009 July and 2010 August. Several sources were observed more than once. In this paper we present the aims of the project, the selection criteria, and the observation and data reduction procedures.