We present a millimetre-wave site characterisation for the Australia Telescope Compact Array (ATCA) based on nearly 9 yr of data from a seeing monitor operating at this facility. The seeing monitor, which measures the phase fluctuations in the signal from a geosynchronous satellite over a 230-m baseline caused by water vapour fluctuations along their sight lines, provides an almost gapless record since 2005, with high time resolution. We determine the root mean square (rms) of the path length variations as a function of time of day and season. Under the assumption of the ‘frozen screen’ hypothesis, we also determine the Kolmogorov exponent, α, for the turbulence and the phase screen speed. From these, we determine the millimetre-wave seeing at λ = 3.3 mm. Based on the magnitude of the rms path length variations, we estimate the expected fraction of the available observing time when interferometry could be successfully conducted using the ATCA, as a function of observing frequency and antenna baseline, for the time of day and the season. We also estimate the corresponding observing time fractions when using the water vapour radiometers installed on the ATCA in order to correct for the phase fluctuations occurring during the measurement of an astronomical source.