Billen was slated to check out of the Desert Palms Resort and catch a flight to Darwin, capital of the NT, on January 5.
She said that Billen was clearly identifiable on footage captured by CCTV cameras outside the hotel on January 1.
"We continue to hope that we'll find her alive but we are gravely concerned about the recent hot weather conditions," Vickery said.
Authorities urged tourists and hikers to exercise caution over the New Year's period with temperatures in Central Australia topping 45 degrees Celsius.
(This story has not been edited by Business Standard staff and is auto-generated from a syndicated feed.)