Complex sentence: The convoy was down to six ships when, at about 1800 on 15 June 1942, when the convoy was south west of Crete, HMAS "Nestor" was straddled by a stick of heavy bombs which caused serious damage to her boiler rooms.
Split into simpler sentences: The convoy was down to six ships when the Australian destroyer HMAS "Nestor" was badly damaged. At about 1800 on 15 June 1942, when the convoy was off the south west corner of Crete, HMAS "Nestor" was straddled by a stick of heavy bombs which caused serious damage to her boiler rooms.