Bethells Beach is located on the west cost of Auckland City - Regional Park site and Wikipedia Reference.
It is a long flat black sand beach with a large cave at the south end and the outflow from the Waitakere River at the north. The river would, I imagine, only be difficult to cross if in flood as it was only knee deep over the sand the day I was there.
The beach has been a very popular filming location for numerous feature films and music videos including for heavy weights such as Taylor Swift and Shania Twain. (see the Wiki ref).
Seems to be very popular with people with dogs and horse riders.
Looking south along the beach at low tide
Looking north from the cave entrance