Saltmarsh in NSW and Australia is shrinking. The cache will take to an area of the river where Sydney Water is attempting to rehabilite the river.
Saltmarsh is a community of plants that can tolerate salty soil and salty water. They are not usually subject to daily tidal flows, but are inundated by larger tides and other events. Saltmarsh is important for the environment as it is a source of food to local wildlife. There are various signs from the trailhead to GZ that explain the significance of the site.
At GZ, you do not need to go beyond the fenced area to retrieve the cache, but do be careful and don't drop it.