Until it shoots or moves, the machine cannot be seen, and cannot therefore be shot at or charged. Once the machine has shot/fired or moved its position is revealed and it can be seen and shot at or charged as normal.
Source: Warhammer Fantasy: 5th Edition
Until it shoots or moves, the machine cannot be seen, and cannot therefore be shot at or charged. Once the machine has shot/fired or moved its position is revealed and it can be seen and shot at or charged as normal.