The 63-year-old's wife contacted police after he failed to return from a 12-kilometre (7-mile) hike at Nourlangie Rock, a far northern escarpment famous for its native rock art.
"The couple are German nationals visiting the area, and it is believed the man went on the Barrk Sandstone Walk and did not take any water with him in the harsh conditions," police said in a statement.
A ground and air search failed to locate the man on Friday evening, and police said rescuers resumed combing the area at first light.
Australia's north is extremely hot in October, with an average daily maximum temperature of 38 degrees celsius (100 F). Dehydration can be rapid and sometimes fatal.