In my experience this is primarily a 737 problem. FWIW I do hear the cabin crew making a PA that the economy toilets are at the back fairly regularly, if not always. Unlike other aircraft the pax at the front of economy can see the J toilet, its just there, so I can understand their desire to use it, rather than walk to the back. There is also the problem of toilet ratios on the 737, fine for the east coast hops, but trans Tasman and Perth become a problem. 737 in Y has 1 toilet per 81 pax, compare that to 332 1 per 49, 333 1 per 45. This means the queue down the back is generally longer, and there is nowhere for pax to queue, the economy queue inevitably gets in the way of the cabin crew down the back. I imagine it is probably easier for the crew to let people use the J toilet than lengthen the queue down the back.