The pilot was assisting in flood rescue operations and was directed to a homestead where several people had climbed onto its roof to escape from the fast flowing flood waters, which had risen to the height of the roof line. He approached the homestead at a height of about 2m above the water level, then hovered the helicopter along side the roof to allow an elderly person to board. This person had had a safety rope attached to him to prevent him from slipping off the steeply pitched roof. As he climbed onto the skids his weight moved the helicopter slightly sideways and the rope snagged on the roof causing the helicopter to descend. The pilot was unable to arrest the downward movement in the height available before the helicopter struck the water and sank. All occupants escaped, the pilot recovered the ELT and activated it.