Mt Gambier, SA
One Tree Sinkhole is one of the major Mount Gambier limestone sinkholes accessible to qualified cave divers. The site is a funnel-style cave with a collapsed roof forming a rock pile at around 30 metres below the surface, with passages descending to 50 metres well beyond CDAA recreational limits. The water is freshwater, cold (around 15 °C year-round) and exceptionally clear. Access requires a CDAA permit and at least CDAA Cave Diver certification; the site is on private property and entry is by booking through CDAA-recognised dive operators. The maximum CDAA recommended depth on air is 40 metres with a 60 metre linear penetration limit.
Visibility at One Tree Sinkhole typically ranges from 30 to 40 metres, though it shifts with swell, wind and recent rainfall. Year-round generally offers the best conditions for diving here.