There's a park between Newcastle and Sydney Australia that has some really impressive sea caves in it, but you can't find it on the net or on the Park's brochures, nor is it marked on any sign. It's at Timber Beach, in the Munmorah State Conservation Area. It's 41 km north of Gosford via Elizabeth Bay Drive off the Pacific Highway. You can get to this beach by going to the second lookout going up the hill from Snapper Point. It's only a ten minute walk down.
Here are some pictures from within the caves:
This is the first sea cave you get to. You can go into this one even when the tide is higher.
This is the second one.
This is the third one which is just around the corner from the second one. This is the passage you have to climb through to get to the fourth cave.
Here's a video of the fourth and biggest cave. There are other passages going off it if you are brave enough to check them out.
There are more pictures here and here.
Note: There are also caves at the next beach over called Ghosties but they aren't as impressive as the ones at Timber beach.
If you do go, bring a strong torch (flashlight) as there are many passages that you can't go into without one. You can only see these caves during extremely low tide (0.2 meters or less) so check the tide tables before you go. Also be careful not to be caught by the incoming tide or you could get trapped in there. I'm guessing this is why the caves aren't publicised. I wouldn't recommend this if you have young children. They might slip or run off into some unknown passage and get themselves lost. Teens on the other hand would love these.
To check the tides go to Ghosties tides.