I think it's this one: posted in Parra forum
6. SANDBAGGING (FLOPPING)
The Code: A DEFENDER CANNOT DROP OR FALL ON A PRONE PLAYER.
Application: It is an infringement for a defender to drop, dive or fall on a player in possession of the ball who is
prone or stationary on the ground and not attempting to play-on. (A simple hand-on completes
the tackle).
Reference: Law Book - Section 15, Law 1 (i), Page 38.
That rule is listed in the rule book as only applying to grades up to u15s
http://www.playnrl.com/wp-content/uploads/2014/02/ARL-Rules-book-2016.pdf