Officers and Kakadu park rangers are searching for Elaine Margaret Maxwell, aged 61, who has not been seen for six days.
"She could have been taken by a croc. It is known that there is a resident crocodile around the boat ramp," said Senior Sergeant Roger Illet of Jabiru police.
The crocodile is about 3.5 to 3.8m long.Ms Maxwell was last seen camping next to her vehicle at South Alligator boat ramp car park on Friday February 24.
Senior Sergeant Roger Illet of Jabiru police said the South Alligator River has been experiencing high tides of over seven metres recently and they have concerns for her safety.
Police and rangers have searched on foot by helicopter and on quad bikes.