This is a NSWGR Signal Relay Hut used to house signalling relays and accompanying batteries.
This version is for those huts placed on the ground as seen in some prototype photos.
These huts also were built on a supporting frame presumably depending on the location. A version on the supporting frame is available here
While normally seen in automatic signalling areas they have been photographed beside lower quadrant signals where the signal lamp is electric, so the lamp is being powered by the batteries in the hut. It would seem that these lower quadrant signals with the huts could be well outside station limits, too far for the Station Assistant to carry the old style kerosene lamps to. This is just conjecture though.