Hey Adrian
From memory the permits are about $16 for the day, Qld parks and wildlife website is where you can get all the info on them. As far as travel times go usually 2hrs either side of LOW tide is the go. Having said that the rainbow side of double island changes all the time, sometimes easily passable and sometimes not with fallen trees and a flat beach meaning the water is up to the foot of the dunes. It all depends on where you are getting on the beach. The stretch up from Noosa is usually always passable at low tide, If your going in at rainbow and the tide is up, you can take the freshwater track which is a couple of K's out of rainbow and will deliver you onto the east beach at freshwater.
Have fun and be aware that there will be cops everywhere up there over easter and enforce ALL road rules.
