Don't know what direction you're travelling, but there's a public dump point out the front of the Caravan Pk at Horsham. Only one I'm aware of at this point.
You will find the same in Echuca too, pay at the caravan park.
Or pop over to Moama to the Golf Club, free overnight stay with dump point.
I am really surprised up here in the top of WA every overnight spot, free, has it's own dump point. We empty before we leave for the next part of the trip. So convenient.