By Rachael Ward

A search for a university student missing in a national park since Boxing Day has entered its sixth day, with fears the experienced bushwalker may not have much food or water.

Hadi Nazari, 23, was last seen by friends when he set off on a bushwalk in Kosciuszko National Park about 2.30pm on December 26, descending the Hannels Spur Trail at Geehi, between Khancoban and Thredbo.

His mates, aged 23 and 24, started looking for him when he failed to turn up to Geehi campground as planned.

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, the National Parks and Wildlife Service, NSW Ambulance, police aerial support and other emergency services joined search efforts the next day in the rugged alpine park.

A helicopter with infra-red capabilities has also been used but failed to locate him.

NSW Police said there were concerns for the man’s welfare, but had been told he was an experienced hiker who had camping equipment and was wearing suitable clothing.

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 Mr Nazari or may have information to come forward.

He is described as being Asian in appearance with a slim build and about 172cm tall.

Mr Nazari was last seen wearing a grey long-sleeve shirt, grey pants and had a blue backpack.