I’m an exchange student from CityU in Hong Kong. We took the advice above and went to Tai Long Wan beach from Pak Tam Au. We were pretty unprepared for the hike and, although it’s a concrete path all the way up it took us about 3 hours to get to the beach (admittedly we were carrying beer, bags and tents) but the views are amazing. You can also just do the downhill trek and catch a boat from a small jetty at the bottom of the hill. If you just want to camp on the beach I advise you to get the boat to the beach from Sai Kung and walk back next day towards Sai Wan and Sai Wan road which took us around an hour.
Beware of the tides. We planned to get the boat next day and couldn’t as the sea was too rough. Also, 4 students on the next beach were swept away by a freak 4m high wave at night and one very sadly died.
Apart from this I can highly recommend this beach for camping there’s a real chilled out atmosphere from other campers (mostly surfers) and the views are amazing. You can hire tents from the two restaurants but they are expensive. Water and other supplies are expensive too but it’s justified as the supplies must arrive by sea. If you want a reasonably isolated place you can’t beat the beach next to Tai Long Wan. It’s a short walk ten minute walk away (on reasonably flat ground) and is where many of the people go to surf after spending the night talking to fellow campers on Tai Long Wan.