Between 1959 and 1961, seventy five _O__ wagons were rebuilt for ballast discharge use. The vehicles selected were from the wagon group built 1915 onwards.
The conversion altered the bottom discharge doors from butterfly "pin catch" type to wheel controlled discharge doors. These doors allowed the discharge rate to be controlled whilst spreading ballast on the move.
The numbers were:
211, 213, 215, 221, 222, 224, 231, 232, 235 - 238, 243 - 245, 250, 256, 258, 263, 265 266, 269, 272, 276, 278 - 280, 282, 284, 287 288, 291, 293, 297, 298, 300, 301, 304, 306 309, 310, 315, 317, 319, 320, 323, 326 - 328 330, 332, 336, 337, 340, 350, 351, 353 - 355 359, 360, 363, 364, 366, 367, 369, 371 - 373 375 - 377, 379, 382, 383, 385, 387
Between 1962 and 1968 the majority of the wagons were reclassed to O without alteration or were converted to _OC_ for glass sand traffic.
In the 1970's ON wagons could be found in limestone traffic from Nowa Nowa to Maryvale Sidings.