They may have been taken off a train due to a mechanical failure until they could be repaired and now they have been repaired, are now being taken back to Kooragang or Port
It looks like the drawbar on the trailing hopper (91453) failed- you can see it dangling in the photo
This was a movent of 3 wagons of a 4 pack of NHWH set being transferred to Branxton for repairs and interior respray. the 4th wagon of the 4 had to be transferred as a separate movement. The drawbar between the 3rd and 4th hopper had broken.