You find yourself at the western end of a 1930s heritage listed sandstone retaining wall.
The wall was constructed to retain the coastal sand dune in the area to help the street above and houses below it to coexist. At the end of the listing is more information about the significance of the wall.
As for the hide, the hint will narrow down the search a lot. If there are muggles in the adjacent park stealth is required and the use of the hint is recommended. The T-rating reflects that if you are all but tall you might need a boost to reach the cache hide (but this should hopefully keep the curious little muggles at bay). For the other half it will be an easy retrieval.
The adjacent park is used as a playground and dog park where you will encounter muggles at various times. I am frequenting the park a lot with my dog. This is recommended during the daytime not to cause suspicion. This is my first cache hide, enjoy.
Here some more information about the wall from the NSW heritage inventory
Significance: The Dolphin Street sandstone retaining wall and embankment are significant as evidence of the early twentieth-century development of civic infrastructure and road building in a sandy and undulating landscape. The sandstone retaining wall has historic associations with the operation of the stone quarries throughout the Randwick municipality and the workers employed in them.
Description: The wall is constructed of irregular sized sandstone blocks with a consistent 20cm high rock-faced capping along the full length of the wall. The blocks have either ashlar or sparrow-pecked finishes. The middle section of the wall around the central stair is made of larger blocks, indicating that this section may have been constructed at a different time to the rest of the wall. The central stair has a curved return and its upper portion is topped by a metal pipe handrail. The western stair has a metal pipe handrail on its lower portion and a timber railing along its upper portion. The wall is terraced at the western end, with two sections overlapping for approximately 15m. The westernmost portion has been partially built over a natural stone outcrop which forms part of the gully at the junction of Dolphin and Judge Streets.
Historical Notes: The retaining walls evident across the Randwick municipality were largely constructed in the first half of the twentieth century as part of the civic infrastructure required by the developing area. As residential development and subdivision increased across the Randwick municipality, the building of new roads and the need to manage the sand dunes and drift that were a feature of the coastal suburbs increased. Building new roads required cuttings to be made through the existing sand dunes and hillsides where the exposed material was not stable enough to stand of its own accord.
Randwick Council constructed retaining walls in areas where roads cut through the former dunes or where sand drift was a problem. As ownership of sandstone in the municipality was vested to the council in the 1860s, this resource was a readily available material for the construction of retaining walls as well as kerbing and guttering. Eleven quarries that cut sandstone blocks for construction and infrastructure works operated within the Randwick municipality at the time. These quarries were operated by a variety of private operators, the state government and Randwick Council, with most operating between the turn of the twentieth century and the 1940s. Some quarries were in operation earlier (St Marks Glebe Quarry in St Marks Road from the 1880s to the 1890s) and others lasted until the 1970s (Story Street Quarry, Maroubra).
Stone used throughout the municipality was most likely sourced from these local quarries. The Dolphin Street retaining wall and embankment appear in 1943 aerial photographs of the suburb, and the adjacent houses appear to date from the 1930s. The wall would have been constructed when the road was formed at the upper level to connect with Judge Street, requiring the construction of an embankment to stabilise the ground and protect the houses below on the northern side of the road.
All information taken from https://www.hms.heritage.nsw.gov.au/App/Item/ViewItem?itemId=2310458