We do have similar symbols here in NZ. They are usually in red so as to denote it is important. If it is very important, it should have an exclamation mark. The anchor
symbol will either be a plain anchor which notes that it is safe to anchor, or a line through it to denote it is not safe to anchor. One such similar symbol as you describe is in my local waters. It denotes a designated safe anchoring
area for commercial
traffic like Barges and Ferries etc within the harbour. They would know there is no cables
etc and other harbour users should proceed through that area at night, knowing a large black wall of the ship maybe there at anchor. They maybe be waiting for another vessel to leave the port or tide or whatever. I even see the big Seatow Barges (huge coal and salt
barges) anchor in that area for weeks. The symbol on my chart gives a swing area as well.