Noumea we should have booked something but we just walked around the harbour to the marina, had a coffee, bought snorkel gear in a small place next door to Quicksilver shop in town (don't buy from the ship its expensive). Mystery Island we did turtle snorkel tour w locals, was wonderful & saw turtles. Lifou we walked across island and snorkelled on our own. Isle of pines just walked down the beach away from people to swim, however sadly there were three local children throwing rocks at people on the beach. Take small AUD notes and everything is well priced or don't buy it if you don't think so.
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