:o
I should have added a link for Qantas. Here it is.
Probably because they’re being fed contraceptives.
Seriously, Cunctator, I once read that a small group of rabbits was put on an uninhabited island near New Zealand once. The rabbits, away from their natural enemies, multiplied rapidly and ate up all the island vegetaton–then starved to death. 
It might be better to transport koalas to areas outside Victoria than to screw up their love life.
My all time favorite is a recombination of similar signs.
“CAUTION! Trespassers will be violated!”
(This thread’s pretty spry for a corpse!)
My favorite is a warning sign a chemist friend brought to the local “party” house. If you can work your way around the drumming hippies, past the assortment of smoking implements and peer through the smoky haze, you’ll see on the refrigerator, in very official black on red:
ALWAYS ADD ACID
TO WATER
(Yes, I know what it means in it’s original laboratory habitat, but it’s so much funnier in the hippie house!)