Ancient Aboriginal rock art in Kakadu has been defaced by visitors, under the noses of tour operators.

The Northern Territory News has learned rock art at Ubirr in Kakadu _ believed to be up to 25,000 years old has been affected.

National Parks director Peter Cochrane said there had been many incidents of damage to rock art at Ubirr.

``There has been defacing of the rock art,'' he said.

``That has involved some level of graffiti.''

``Mr Cochrane said investigations suggested the damage had been done by people brought into the park by tour operators.

````The great majority of tour operators behave responsibly and the situation is improving,'' he said.

````However, there are a small number of operators who do give us problems from time to time.''

``He said other instances of tour operator misbehaviour included:

``HANDLING of wildlife, a situation brought to light after a freshwater crocodile was reportedly ``provoked and hassled'';

``AT LEAST two tour operators had their permits to access the top of Twin Falls revoked following safety concerns about their operations;

``A VISITOR was found in banned sacred sites at Koolpin Gorge; and

``GERMAN tourist Isabel von Jordan, 24, was killed by a 4.6m crocodile after being told it was safe to swim by her tour guide.

``Mr Cochrane said the damage was a consequence of up to 160,000 people visiting the park each year.

````There is always going to be some kind of tail end of people who don't show or feel the same sort of respect that the vast majority do,'' he said.

``Since the graffiti incident, park staff are always at Ubirr when the site is opened.