Kinda agree with Lost here - as soon as you make something fool-proof, someone goes and makes a better fool.
What about the opposite approach - a pair of high-def video cameras on the sides of the entrance track, hooked up to a motion detector? Rig up a basic speed-sensor trigger (a pair of inflatable tubes like the road counting machines use) so you don't have to sift through footage of bushwalkers coming through. Review footage weekly, pass on photos + day/times to the police. If you notice a pattern (every Friday at 6pm, say), the 5-0 might be able to sit there one day and wait for them. If you want to be nice, stick up a big sign explaining why no trail bikes are allowed, so that the riders don't think they're just being arbitrarily locked out.
Alternatively, how heavy are these kayaks? Rather than having a hole for the kayaks to go under the fence, are you able to fit a boat roller to the top of the fence? Kayak person walks up to fence, puts kayak on ground, lifts nose up and rests it on roller. Person walks through turnstile or other bike-unfriendly device. Person pulls kayak over the fence, aided by the boat roller. It's a bit messy, and unless you've got an unbroken heavy-duty fence that runs off each side of the gate then they're probably just going to ride around it.
That's my $0.02. It's worth considerably less than you paid though.
Cheers,
Matt
