This large sea cave is one of my favorite spots on the Auckland west coast and definitely a must-see when you visit Bethells beach.
It's located on the southern end of the beach, and is accessible a couple hours either side of low tide. It's a perfect spot for photos during golden hour, just make sure you check the tide times before you go. There is also a smaller cave nearby to explore.
Some extra info:
Distance: Approx 2km from the carpark
Walk time: Approx 20 minutes
Note: Carpark is open 7am-9.30pm (summer) and 7am-7pm (winter)