When I was growing up in the 1960s, our family would holiday at The Entrance and I fondly remember the old wooden bridge with its steps partway across leading down onto a substantial island, as seen in this aerial photo from around that time.
When the new bridge was built just west of it in 1969, steps down to the island were also included, however the shifting sands in the channel, possibly associated with dredging, caused the island to gradually disappear. The satellite view from 2007 shows part of this island east of the bridge and a remnant at the base of the steps, but now both have completely gone.
You can still walk down the steps, although the bottom landing is now fully enclosed as it sits above deep water and it has become a popular platform for anglers.
The listed coordinates are at the top of the steps. which consist of two flights separated by an intermediate landing. To determine the location of this cache, you need to count the steps in each flight (just the steps, not the landings above or below). Multiply those two numbers together to yield a three-digit number X.
X = (top flight steps) × (bottom flight steps)
As a quick check, the sum of the three digits of X is 11.
The cache is located at:
S 33° 20.0(X-161) E 151° 30.(X-11)
This is nearby alongside what I suppose is an anti-island - a body of water surrounded by land - and the container a grey plastic tube hanging from a tree branch at about head height.
My thanks to lee737 who kindly 3D-printed the containers for this series.