For people who wish to get there. It is compulsory to have a Permission letter from the Forestry Department. They have a office somewhere in Jakarta on Gunung Sahari Road road ahead towards Senen. Sorry but I lost the contact Details for the same.
Getting permission is free of Cost.
However in Indonesia without "Uang Roko" nothing is free.
So u might need to bribe the guys with some small money like 50,000 Rp to get the permission.
Make sure you carry Originals with you.
Do not worry the People are nice and co-opeartive.
It's not a dangerous case but take it as a reference. When landed at Onrust Island, someone will ask you to pay the entrance fee. Rp 2,000 per pax. Yes, it's legal and not expensive or in other words; very cheap. But sometimes, the ticketing guy didn't give the ticket [a green one], only showing them and say the tickets are limited. He also asking; you need a local guide or not but never insistent.
Unique Suggestions: For me, it's okay :) But I check the stamp first to see it's legal or not [using the emblem of DKI Jaya]. And if you don't need assistance, please say "Tidak perlu, terima kasih." And he will understand. Do not leave without a word.
Fun Alternatives: You can ask to have the ticket or do like I did; check it carefully and be sure.