Darwin to the Tiwi Islands, where the sailing day sets your time ashore and a permit decides how far you can go.
The Northern Territory route on this site is SeaLink's ferry from Cullen Bay in Darwin to Wurrumiyanga on Bathurst Island, one of the two Tiwi Islands. It sails only three days a week, and the length of your stay ashore depends on which day you pick: a long one on Thursdays and Fridays, a much shorter one on Sundays, when the service also carries on to Paru on Melville Island.
The Tiwi Islands are Aboriginal freehold land, and the Tiwi Land Council controls access by permit. Coming over on the ferry for the day, or with a hosted tour, needs no application, because a day pass is issued instead; leaving the community boundaries does need a permit, applied for well ahead. Alcohol is banned on the islands, luggage is screened at Cullen Bay and biosecurity officers work the wharf, so read the route page before you pack.