This shot made by Pablo Carreño Busta (Spain, age 30) in his five-set match against Tallon Griekspoor (Netherlands, 25) at the Australian Open is perfectly legal but very rarely seen.
Busta won the match after 4 hrs 10 mins of play.
You are not allowed to touch the net, or to jump over the net to play a ball, to return a shot. You can run around the net post and play from there, provided your feet/ your racquet does not touch the inside of the opponent’s singles court.
It is OK to reach over the net to hit a shot, while not touching it, nor touching the inside of the opponent’s singles court lines.