Kilsby Sinkhole is the one Mount Gambier sinkhole that ordinary Open Water divers can reliably get into, and it is the site most visiting divers remember. The hole is a large collapse doline in Oligocene limestone, walled by vertical grey cliffs hung with ferns and ivy, on a sheep property that five generations of the Kilsby family have farmed. From the pontoon the water drops away in a sheer, clear column – visibility of 50 m is normal and it is often more, so you see the whole bowl of the sinkhole from the surface. Below the lip a slope of rock and silt leads down into the main basin, and a crystal-clear flooded chamber opens off the south-western side, where daylight fades into blue. The upper few metres carry filamentous algae and aquatic weed that glow green in the midday sun, and light shafts pierce the water between about 11 am and 2 pm.

The site has an unusual history. It began as a stock watering hole, was one of Mount Gambier's pioneering dive sites in the 1960s, and was closed after a drowning in 1969. It reopened to civilian divers in 1983, has been used as a training site for SA Police divers and – according to the owners – as a centre for secret weapons research, with the depth and clarity making it ideal for underwater testing. A 2007 lease between the family and the CDAA extended access times and permitted deeper dives on trimix.

Today the Kilsbys also run snorkel tours, dry sinkhole tours, accommodation and gin tastings, so expect other visitors on the rim. For Open Water divers the dive is essentially an introduction to sinkhole diving in a controlled, daylight setting – neutral buoyancy over a bottomless-looking drop is the skill it teaches. CDAA Basic Cave divers use it for deeper, longer dives and photography, and technical divers use it for trimix training. It suits anyone with good buoyancy who wants to see what 'gin-clear' actually means.

From Mount Gambier drive south about 14 km (around 15 minutes) towards Moorak/Mount Schank; the property is on private farmland and the exact address and gate instructions are given only on booking. Park at the designated area near the sinkhole rim, gear up on the grass and carry equipment down the formed stairway to the pontoon at water level – allow for several trips with twin sets or stages. Open Water divers must come with one of the licensed operators (Dive Experience of Mount Gambier runs Wednesday and Saturday sessions; Diving Adelaide and several Melbourne operators run trips).

CDAA members book their own slot through the CDAA Kilsby booking system (cdaa.kilsby@gmail.com) – each diver books only themselves.

Weather has little effect underwater – there is no swell, current or tide – but surface conditions matter for the long gear carry and surface intervals. After very heavy winter rain surface run-off can cloud the upper few metres. Summer brings the best light shafts around midday and algae growth in the shallows.

Private and forestry land may be closed on declared Total Fire Ban days (common December-March), so check the CFS fire-ban forecast the night before. Groundwater levels in the Lower South East have been falling, which has caused closures of the DEW ponds sites in recent seasons. Kilsby itself is sheltered by its walls and is diveable in almost any weather.

Descend from the pontoon down the sloping wall to 15-20 m, then swim out over the open lake to the mouth of the south-western chamber, staying within the daylight zone and the depth limit of your grade (27 m for Open Water with operators; 40 m Basic Cave; deeper on trimix under the CDAA lease). Circle the chamber wall, ascend along the opposite wall and do the safety stop on the weed-covered ledges near the pontoon.

CDAA grade: Basic Cave (5L46) for independent member access; Open Water divers only with licensed operators within the 27 m open-water lake zone, no decompression. Deeper dives on trimix permitted under the 2007 CDAA lease conditions. No overhead penetration for OW divers; the south-western chamber should only be entered within grade.

Silt on ledges – keep off the floor. Rebreathers welcome with CDAA grade.

Licensed operators only for non-CDAA divers: Dive Experience (Mount Gambier) – guided Open Water dives to 27 m on Wednesdays and Saturdays, often paired with Ewens Ponds; Diving Adelaide – regular Mount Gambier weekends and CDAA courses; Deep Blue Ventures (Melbourne), Blue Water Ascent and Total Immersion Diving also licensed. CDAA members dive under a lease agreement via CDAA booking; freediving only with licensed freediving instructors.