I often saw Nerita atramentosa (Black Nerite) clustered like this between rocks and in tidepools. They look like black olives, and are very common on Lord Howe Island.
This Scutus antipodes (Elephant Snail) was the only one I saw on Lord Howe Island. Many thanks to naturalist Ian Hutton for noticing it and showing me.
This bumpy snail was the only one I saw during low tide at Lord Howe Island. Based on information from the LHI Museum site, I think it's MULBERRY SNAIL (Morula marginalba).