The family of missing German tourist Monika Billen have offered some new clues about the movements of their "dearly beloved daughter, sister and aunt" in the days before she disappeared.

Key points: Billen family have sent letter imploring witnesses to come forward with new information on relative's disappearance

Billen family have sent letter imploring witnesses to come forward with new information on relative's disappearance Letter reveals Billen had been enjoying walks that offered "plenty of shady places to sit, dream and read"

Letter reveals Billen had been enjoying walks that offered "plenty of shady places to sit, dream and read" Billen missing in Central Australia since New Year's Day

In a letter to the ABC, Ms Billen's family have shared some extracts from the last email the intrepid hiker sent.

It mentioned taking "more or less extensive walks in the surroundings of Alice Springs".

It is now two weeks since Ms Billen, 62, was last sighted in Alice Springs on New Year's Day.

Police have said they believed Ms Billen went hiking on the 43.5 degrees Celsius day.

But after five days scouring the town and nearby hiking trails, using helicopters, drones motorbikes, dogs and horses, police called off the search for the woman on Sunday.

'Travelled off the beaten path for years'

Ms Billen's family in Cologne have written a letter imploring the Alice Springs public to come forward with any information which could lead to her discovery.

"We fear that Monika has had an accident or has been the victim of a violent crime," the letter read.

Police distributed this dated photograph of missing woman Monika Billen. ( Supplied: NT Police / Facebook )

"Monika has travelled off the beaten path for many years and in many countries and has always proceeded with caution.

"It is normal for her to take several days between messages to us, and that is why we did not know at first that she was missing.

"We have been consumed with worry ever since we heard of Monika's disappearance, especially because we know her as a very responsible and capable person."

Ms Billen wrote of walking in heat

Ms Billen was last seen by staff at the Desert Palms Resort, where she had been staying for more than a week.

But it took more than week for a search to begin, with the resort staff waiting three days after she failed to check out on January 5 to report her disappearance to police.

Police found she had also missed a scheduled flight to Darwin on January 5.

Monika Billen had been travelling alone in Australia when she disappeared. ( Supplied: Facebook )

During a search of her room they found brochures for local walks, including the 223-kilometre Larapinta Trail, leading them to believe she had gone hiking.

In her last email to her family on December 31, Ms Billen detailed her plans to walk the region.

"In the heat I take more or less extensive walks in the surroundings of Alice Springs in Central Australia," Ms Billen wrote to her family.

"Somehow the heat fits well with the landscape … I took a picture from the [Olive Pink] Botanical Garden lookout hill, which is near my accomodations (sic) and offers plenty of shady places to sit, dream, and read."

Family urges witnesses to come forward

Several appeals for public assistance did result in multiple tips, which led to the focus on Emily Gap.

Police said that was the location of her last confirmed sighting, despite reports she had been picked up by a local resident and dropped off at the Olive Pink Botanical Gardens on January 2.

Map Monika Billen had been staying at an Alice Springs motel and is believed to have visited Emily Gap, east of the town, shortly before she disappeared.

"We are trying to clarify some information in relation to the timing that Monika was dropped off at Olive Pink," NT Police Acting Superintendent Pauline Vicary said on Friday.

"That may have actually been earlier than anticipated now."

On Friday, police said they had footage of eight cars driving past Ms Billen as she walked down Barrett Drive on New Year's Day, but none of those drivers had contacted them.

Despite her vanishing seemingly without a trace, police said they did not believe Ms Billen's disappearance was suspicious.

Ms Billen's family have thanked the NT Police "and those who have been helping them search for Monika".

"[We] urge you to call them with any kind of information that can help bring Monika back to us, please call 131 444 or write a mail to findmonika@web.de."