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We present first results of the H 2 O Southern Galactic Plane Survey (HOPS), using the Mopra Radio Telescope with a broad‐band backend and a beam size of about 2 arcmin. We have observed 100 deg 2  of the southern Galactic plane at 12 mm (19.5–27.5 GHz), including spectral line emission from H 2 O masers, multiple metastable transitions of ammonia, cyanoacetylene, methanol and radio recombination lines. In this paper, we report on the characteristics of the survey and H 2 O maser emission. We find 540 H 2 O masers, of which 334 are new detections. The strongest maser is 3933 Jy and the weakest is 0.7 Jy, with 62 masers over 100 Jy. In 14 maser sites, the spread in the velocity of the H 2 O maser emission exceeds 100 km s −1 . In one region, the H 2 O maser velocities are separated by 351.3 km s −1 . The rms noise levels are typically between 1 and 2 Jy, with 95 per cent of the survey under 2 Jy. We estimate completeness limits of 98 per cent at around 8.4 Jy and 50 per cent at around 5.5 Jy. We estimate that there are between 800 and 1500 H 2 O masers in the Galaxy that are detectable in a survey with similar completeness limits to HOPS. We report possible masers in NH 3  (11,9) and (8,6) emission towards G19.61−0.23 and in the NH 3  (3,3) line towards G23.33−0.30.

The H 2 O Southern Galactic Plane Survey (HOPS) – I. Techniques and H 2 O maser data