Melbourne student Hadi Nazari, 23, disappeared after setting off on a bushwalk in Kosciuszko National Park about 2.30pm on December 26.
His friends last saw him as he descended the Hannels Spur Trail at Geehi, between Khancoban and Thredbo.
They started looking for him when he failed to turn up to Geehi campground as planned.
Search crews have been unable to find any trace of Hadi Nazari in Kosciuszko National Park. (Phil Barton/AAP PHOTOS)
Authorities were alerted just before 9pm that night, with local police and State Emergency Service personnel launching a search on foot and by vehicle.
Specialist rescue crews, NSW Ambulance, police aerial support and other emergency services joined search efforts the next day in the rugged alpine park.
NSW Police Acting Superintendent Jillian Gibson said Mr Nazari did not have a personal beacon, but was carrying a reasonable amount of water and camping gear and had done similar hikes overseas.
She said crews were battling hot conditions and scaling the area was difficult but volunteers would help with the search into next week if needed.
"Obviously we've got concerns for Hadi, but we also have concerns about our staff, so that's why we have medical teams on stand-by at the command post," she told reporters in Wagga Wagga on Tuesday.
Mr Nazari's sister said the anxious wait for news has left her wanting to "go outside and scream".
"This is the biggest trauma of my life ... not seeing him for so many days, not able to talk to him for so many days," Zahra Nazari told Nine News.
She feared her brother didn't have a lot of food and water as he only planned a day walk.
"I just look at his pictures all the time and pray to God that he come back, he come back soon," she said.
Police are appealing for anyone who saw him to come forward.
Mr Nazari is described as being Asian in appearance with a slim build and about 172cm tall. He was last seen wearing a grey long-sleeve shirt, grey pants and had a blue backpack.
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It comes as a bushwalker aged in his thirties fell to his death in southern Tasmania, becoming the third man to die on a bushwalk in the state over the Christmas holidays.
Tasmania Police said it's understood the Victorian man fell from a height at Federation Peak in the Southwest National Park about 11am on Monday.
He had been travelling with his partner and another woman, with the pair activating a personal beacon which prompted emergency services to quickly start searching for the man.
His body was recovered from the area about 9am on Tuesday.
The women were not physically injured and spent the night in bushland before being airlifted out of the park.
Inspector Colin Riley says Federation Peak is one of Tasmania's most challenging walks. (Ethan James/AAP PHOTOS)
Tasmania Police Inspector Colin Riley described Federation Peak as "probably one of the most challenging walks we have in Tasmania" and said all three were relatively experienced bushwalkers.
"This area is steep, very narrow passages and you effectively have to crawl your way up towards the summit," he said.
Two men were also rescued from Federation Peak on Christmas Eve, while Insp Riley attributed the recent deaths of two other bushwalkers to medical episodes.
He urged anyone setting out on a bushwalk to take personal locator beacons and adequate equipment, and to stop hiking if conditions changed and they didn't feel confident.
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