Hopes are dwindling that a missing hiker will be found alive as experts warn of the rough terrain and scarce water availability.
The 23-year-old hiker, Hadi Nazari, was descending a challenging trail in the Kosciuszko National Park about 2.30pm on Boxing Day when he was last seen by friends, who raised the alarm when he did not arrive at the campground where they arranged to meet.
Since then, NSW Police, the National Parks and Wildlife Service, the State Emergency Service, NSW Ambulance and volunteer rescuers have been searching for Mr Nazari.
Search and rescue specialist Paul Luckin, who assisted police with the search, said that of the 105 searches he's been involved in over the past 12 months, just three people have been found alive after long periods alone in the wilderness.
"We don't have many who are missing for an extended period and come out alive," he told AAP.
"Ultimately, his survival is going to depend on finding drinkable water and probably staying at that water."
Dr Luckin said the availability of water in the area where Mr Nazari went missing was scarce.
"Whether you can find water or not depends on the terrain and how thick the vegetation is, it is well known that the vegetation in that particular area is very dense, very thick vegetation," he said.
"There is some water (there), but accessing it is difficult"
Rubbish and hiking poles believed to belong to Mr Nazari were located late on Tuesday but he is still yet to be found.
He was not carrying a personal locator beacon but was thought to have a reasonable amount of water as well as camping gear, police have told reporters.
Dr Luckin said injuries could also be a factor in influencing whether Mr Nazari would have survived the week.
"If somebody has a minor injury and they're still mobile, then you would expect that they would be able to take steps to get themselves out of the situation by making themselves visible, by signalling or by responding to calls," he said.
"With a major immobilising injury it comes down to whether they can be found or whether they can reach water before they die of dehydration."
A helicopter with infrared imaging technology has also been used in the search and specialist crews have been flown into the rugged terrain.
Mr Nazari was last seen on the Hannels Spur Track between Khancoban and Thredbo in the national park.
It is regarded as one of Australia's most challenging bushwalking trails, with an 1800m change in elevation.
"It's part of the most rugged terrain in NSW, it's very steep, it's very vegetated and it's quite remote," NSW Police acting Superintendent Jillian Gibson told reporters earlier in the search.
Dr Luckin said police throw "enormous" amounts of resources at search and rescue operations.
"They will often continue for quite a lot longer than I suggest that people might survive," he said.
"People sometimes think (they) will stop because it costs too much, but that is exactly the opposite of the case, they will search beyond what you would think a reasonable prospect for survival."
