RS&BBM
I don't think it specifically has to be a rabbit,
if this is true then any decomposing creature may do. Perhaps this explains why that cesspit in Kempsey had been functional for all these years.
If he had dug another pit, connected to the old one or at a different location with new pipes, the bag would possibly never have been discovered.
When our area was connected to the sewage system, my old neigbour kept his cesspit and simply had the overflow connected to the big sewage pipe. (We never suspected him of hiding anything and no one here is missing.) Earlier on, someone asked if it was customary in rural areas to hide bodies in cesspits or septic tanks. IMHO it is not, although it happens and chances of discovery are high, unless you play it smart and then you might get away with it for many years.
My own cesspit was a bit of a mystery. Some years it would fill up with sediment, but when we opened it after the house had been connected to the sewage system, the disconnected pit was clean and empty.