I'm using the standard fps character controller and have a simple fly through script attached to it..

// fly script

var change:double = 3; // Determines how quickly you move 

private var targetPosition:Vector3;

function Update () {

        if (Input.GetMouseButton(0)) {
        transform.position = transform.position+Camera.main.transform.forward
*change ;
        }
        if (Input.GetMouseButton(1)) {
        transform.position = transform.position-Camera.main.transform.forward
*change ;
        }

        if (transform.position.y <0) {
        transform.position.y = 0;
        }       }

It works well, and I can fly around the scene but am unable to go below zero. How would I adjust this? I have tried setting gravity to zero in the controller settings but still cannot fly below zero.