Parabellum, you're half right. There are a number of haka, but they are usually tribe-specific rather than occasion-specific. Old protocols dictate (in many cases) only certain members of a tribe can be "called" to perform a haka. The most commonly heard haka, "Kamate, Kamate", actually belongs to one tribe. They have given permission to certain teams, All Blacks to name one, to use it as a challenge. Not being of that particular tribe, I observe the protocol and don't/won't perform it if asked to do so. Essentially, if I did perform it, I would be seen as a traitor to my tribe.
There's a lot more to the haka than meets the eye/ear.