The Sisters are a pair of sinkholes on Sisters Road at Mount Schank, separated by a natural land bridge and managed by DEW. The CDAA calls them 'two separate holes with landbridge' and one of the region's popular training sites, and they are booked through the same Parks SA system as Goulden's Hole.

The two holes have different characters. The northern sinkhole usually has the better visibility, particularly along its wall, and is the place for relaxed, clear-water dives and skills practice. The southern hole is the curiosity: its silty floor holds a collection of dumped objects – a car body, a truck, a cash register and a bicycle – left from the decades when the region's sinkholes served as convenient rubbish pits. Finding them in average visibility is a mini navigation exercise, and the fine silt means the first team in gets the best view.

Depths are modest by Mount Gambier standards and the whole site is in daylight, which is why instructors like it for Basic Cave courses: gas-sharing, line work and lights-out drills can be practised without an overhead. Booking conditions mirror Goulden's – maximum 12 divers per slot including instructors, minimum two divers per booking, 48 hours' notice, two slots per day maximum and an indemnity for everyone. The one quirk is the CDAA's warning about highly aggressive bees in warm weather, so summer dives are best done early. It suits CDAA Basic Cave divers and trainees. Its closeness to Mount Gambier makes it an easy second dive on a Goulden's day.

Book through parks.sa.gov.au (listed under Piccaninnie Ponds Conservation Park) at least 48 hours ahead, with a signed indemnity per diver and CDAA verification. Drive south from Mount Gambier towards Mount Schank and turn onto Sisters Road; the twin sinkholes are beside the road. Park as directed, carry gear the short distance to the edge of the northern or southern hole.

Two DEW timeslots a day: 6 am-noon and noon-8 pm.

Unaffected by swell or tide – the limiting factors are access and surface conditions on private farmland. Summer brings heat, snakes and (at some sites) aggressive bees; Total Fire Ban days can close forestry and some private sites and ban compressors; winter rain turns paddock tracks muddy. Shallow water in sinkholes grows algae in summer, reducing visibility in the top few metres; the water below the thermocline is usually clearer and a constant ~15°C.

The CDAA warns of highly aggressive bees in warm weather at this site – dive early in summer.

Dive the northern hole along the wall where visibility is usually better, then (on a second dive) the southern hole to see the dumped car body, truck, cash register and bike on the silty floor. Stay within the daylight zone and off the bottom.

CDAA grade: Basic Cave (5L43/44), owner DEW. Book via parks.sa.gov.au ≥48 h; two slots daily; max 12 per slot; min 2 per booking; max 2 slots/day; indemnity required. No overhead; stay in daylight zone.

Popular CDAA training site – used on Basic Cave courses by Diving Adelaide, Deep Blue Ventures and CDAA instructors; CDAA members on DEW bookings.