The amphibious aircraft was being flown by an instructor who was training another pilot on the aircraft type. His initial intention was to evaluate surface conditions on Brisbane Waters before conducting training in water operations. The prevailing weather conditions were fine with a reasonably strong north-easterly air flow. The instructor said that the effect of the north-easterly wind was evident from some whitecaps and windlanes in the open water areas. There was a large area in the lee of some high ground which had relatively calm surface conditions. The instructor spent some time discussing the conditions with the student before selecting a suitable circuit pattern and touchdown area. An into-wind approach was flown by the instructor at a speed of 60 kts. The touchdown was normal at about 45 kts, however, when power was reduced at 35-40 kts, after travelling about 40-50 m, the aircraft suddenly rolled left and pitched nose-down. The nose of the aircraft dug into the water causing the aircraft to overturn before coming to rest. Both occupants were able to escape with minor injuries. The aircraft was salvaged and transported to Bankstown where it was examined for any evidence of structural failure. The damage found was consistent with water impact. Although there was substantial structural damage, the hull had remained intact. The instructor concluded that the aircraft had been subjected to a sudden wind gust associated with "down wash" from nearby high ground. This had resulted in the left wing-tip striking the water with an associated nose-down pitch.