The WOA class of thirty-nine wagons numbered 31302 - 31340 were built in 1970. In 1984 nine wagons were sent to South Australia where their bogies were hired to Australian National until March 1985. In 1986 twenty of the class were written off and "used for WHA wagons". Fourteen of the class were converted to WOAS for superphosphate traffic which was quickly changed to WOS in 1989/90 (three going straight to WOS).

1984	WOA 31302, 31305, 31313, 31315, 31320, 31329, 31331, 31338, 31339 to South Australia until March 1985.
1986	WOA 31303, 31306, 31307, 31311, 31312, 31314, 31316, 31318, 31319, 31321, 31323, 31325, 31327, 31328, 31330, 31332, 31333, 31336, 31337, 31340 written off and "used for WHA wagons"
1988	WOA 31305, 31309, 31320, 31338 to WOAS
1989	WOA 31308, 31310, 31322, 31324, 31326, 31334, 31335 to WOAS WOA 31315, 31331, 31339 to WOS WOAS 31305, 31309, 31326, 31335, 31338, to WOS
1990	WOAS 31308, 31310, 31315, 31320, 31322, 31324, 31334 to WOS
1994	WOS 31305, 31308, 31309, 31310, 31315, 31320, 31322, 31324, 31326, 31331, 31334, 31335, 31338, 31339 converted back to WOA
In June 2010 the seventeen remaining wagons were: 31302, 31304, 31305, 31308, 31309, 31310, 31313, 31315, 31320, 31322, 31324, 31326, 31329, 31331, 31334, 31335, 31339. WAGR official photo of WOA31304 (RHWA collection) WOA Diagram