The H2O Southern Galactic Plane Survey (HOPS) has mapped a 100 degrees strip of the Galactic plane (-70 degrees > l > 30 degrees, |b| < 0 degrees.5) using the 22 m Mopra antenna at 12 mm wavelengths. Observations were conducted in on-the-fly mode using the Mopra spectrometer (MOPS), targeting water masers, thermal molecular emission and radio-recombination lines. Foremost among the thermal lines are the 23?GHz transitions of NH3 J,K = (1,1) and (2,2), which trace the densest parts of molecular clouds (n > 104?cm-3). In this paper, we present the NH3?(1,1) and (2,2) data, which have a resolution of 2?arcmin and cover a velocity range of +/- 200?km?s-1. The median sensitivity of the NH3 data cubes is sT mb =0.20 +/- 0.06?K. For the (1,1) transition, this sensitivity equates to a 3.2?kpc distance limit for detecting a 20?K, 400?M? cloud at the 5s level. Similar clouds of mass 5000?M? would be detected as far as the Galactic Centre, while 30?000?M? clouds would be seen across the Galaxy. We have developed an automatic emission finding procedure based on the Australian Telescope National Facility (ATNF) duchamp software and have used it to create a new catalogue of 669 dense molecular clouds. The catalogue is 100 per cent complete at the 5s detection limit (Tmb = 1.0?K). A preliminary analysis of the ensemble cloud properties suggests that the near-kinematic distances are favoured. The cloud positions are consistent with current models of the Galaxy containing a long bar. Combined with other Galactic plane surveys this new molecular-line data set constitutes a key tool for examining Galactic structure and evolution. Data cubes, spectra and catalogues are available to the community via the HOPS website.