The pilot reported that as the aircraft decelerated through 60 knots on the landing roll, braking efficiency was reduced. His attempts to ground-loop the aircraft were to no avail and it overran the strip, while travelling at about 15 knots, and collided with a ditch. The strip is 750 metres in length with a dirt centre section and grassed outer sections. It was reported that rain had been falling all the previous night and had only ceased a short time before the attempted landing. The centre section had had puddles of water on it and the outer section was apparently covered in standing water. This report was compiled from information provided by the pilot of VH-DTV.