The operator and pilot reported that the helicopter struck a single-wire-earth-return (SWER) power line at the beginning of a swath run during a spraying operation. The orientation of the cane paddock being sprayed was north-east / south-west with a major power line running through the north-eastern corner. A SWER power line running east -west joined the main power line over the paddock. The pilot had sprayed the western edge of the paddock, flying the helicopter over the main power line on each spray run. As the pilot moved his spray effort further east on each swathe run, he concentrated on missing and flying over the main power line. On one pass he lost awareness of the SWER line and during descent onto the crop the mast of the helicopter struck the line. The helicopter rolled into the crop. The pilot was not injured