Although you do not need to acquire permits in advance, it may be a good idea to do it if you are planning to come during holiday seasons.
If you cannot get a permit in advance, camp rangers will collect the money at the site but they may charge you $1-$2 dollars more per person. If this is the case, make sure you have cash on hand as the rangers will not accept credit card payments.
If you wish to acquire permits in advance, you can download the application online through The Kauai County Website. Fill out the application, then mail the completed form to:
Department of Parks and Recreation
Park Permits Section
4444 Rice Street, Suite 105
Lihue, HI 96766