Sevak Simonian, 21, has not been seen since he left his parents' home at Belrose in Sydney's north about 8pm on October 20.

His parents reported him missing to police, and his friends then came forward saying he was headed to the Kanangra-Boyd National Park near Oberon.

His car was found in the Kanangra Walls Road car park on October 24.

Police on Wednesday said they believed Mr Simonian entered the park on October 21 but only had a day pack with him when he disappeared.

Mr Smith said he was alarmed when he discovered on Tuesday morning that Mr Simonian had still not been found.

He spent two hours on Tuesday searching the national park in his helicopter.

He said that, from speaking to police, it is understood 25 skilled bushwalkers were helping with the search effort but he said there needed to be more.

"The best way to find someone who is lost in the bush is to have lots of people walking in the bush. If you're a bushwalker, a canyoner or a rock climber, get in touch with the police and offer the fact that you could go and also for bosses to give them the time off," he said.

"Police are concerned about people going in there not skilled but there are hundreds of very skilled bushwalkers in Sydney and they are not there because they don't know anything about it."

Mr Smith said that, while Mr Simonian had been missing for eight days, he believed Mr Simonian could be alive because there was plenty of water about.

Barry Washington, a friend and colleague of Mr Simonian at Bunnings in Narrabeen, said Mr Simonian loved the outdoors, and had been bushwalking on his own since he was 13.

"His passion is bushwalking, that's what he does. He loves it out there, he'd live out there forever if he could. Sometimes he stays out at Barrington Tops [National Park in the Hunter Valley] for two weeks, he likes to live off the land," said Mr Washington, who has been part of the team searching for Mr Simonian.

"He knows how to survive out there, to build shelters. He knows what he can and can't eat and drink."

Mr Washington said that, while Mr Simonian had previously been away on bushwalking trips for weeks, he would always let his employer know that he would be away. He never missed turning up to work, Mr Washington said.

"He never, ever does not come into work without calling in sick. He's responsible with work," Mr Washington said.

"He is a very fit guy, he is very health conscious. He's an organic eater, doesn't smoke or drink; he is one of the fittest people I know and strong as an ox."

Mr Washington said it was a "miserable day with low cloud cover" when Mr Simonian disappeared, and he may have lost his way or decided to take shelter to wait out the weather.

"My opinion is that if he is not injured and he has made it to a creek, he will last for an immense amount of time.

"If he is injured, he is capable of still getting out of there. He's the sort of guy that wouldn't panic," he said.

Mr Washington said Mr Simonian was not depressed, and had recently spoken about his plans for the future.

"He was saving up to buy a farm. He wants to live in the bush. He's just a good lovable kid. A very quiet guy, he likes to keep to himself. He would have gone out there for an adventure, I reckon, and the fog cover's gone too low," Mr Washington said.

Mr Simonian is described as being about 180 centimetres tall, with a thin build, olive skin, short black curly hair and brown eyes.

Mr Washington urged volunteers who want to help in the search to contact Oberon police station.