By Neve Brissenden

Experts hold concerns for a young and experienced hiker still missing in dense bushland as the search and rescue effort enters its ninth day.

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.

Specialist bushwalkers from Sydney’s SES team joined the search effort on Thursday.

Search and rescue specialist Paul Luckin, who assisted police with the search, said that of the 105 searches he had joined in the past year, just three people had 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.

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.