WITH $150 in his pocket and some basic camping equipment, teenager Matthew Allen left home after hatching a plan to run away.

He told his driving instructor not to pick him up the next day because he was going to a party.

Instead the 18-year-old headed into thick bushland near his home in Westleigh, in Sydney's northwest, leaving his mobile phone and most of his clothing at home.

For the next nine weeks Matthew lived in the bush and drank from a muddy creek. He is thought to have packed only minimal food supplies.

An extensive police search at the time of his disappearance on November 27 failed to find any trace.

However his life in the bush, during which Sydney sweltered in record 45C-plus heat, came to an end when two hikers spotted a "disorientated-looking" figure near a creek on Saturday.

After alerting police, emergency crews went into the Berowra Valley Bushland Park and found him near a tree. He was severely dehydrated, having lost about 40kg - half of his body weight - and his limbs were covered in leeches.

One of his legs had been injured and become gangrenous, as had some of his fingers.

"He was in a pretty shocking state. He was lying down, disoriented and exhausted. He was also partially blind," Detective Inspector Glyn Baker said.

The paramedic who helped winch him to a waiting ambulance yesterday told of the delicate rescue operation. Special casualty access team paramedic David Zids said the teenager was located 200m behind homes but his location shielded him from view.

"It was a 400m walk in difficult terrain to reach the patient and he was found sitting near the creek," Mr Zids said.

"The patient was assessed. He had lost a lot of weight and was covered in bites from leeches and mosquitoes. Because he was able to walk with assistance, we helped him walk up out of the creek line - where we couldn't winch - to a position on the ridge line where he could be winched.

"He was fairly weak from malnourishment. He'd only been drinking out of the creek, which runs off an urban area."

Exactly how Matthew, an accounting student at Castle Hill TAFE, survived in the extreme weather will remain a mystery until he is well enough to talk, police said.

The Daily Telegraph has been told he has mental health issues and wanted to "run away from his life".

He is being cared for in a psychiatric hospital, as well as receiving treatment for exposure and dehydration.

Matthew's mother Deborah said, while the family was "relieved" he had been found safe, her main concern was his recovery.

"We just want to get him well," Mrs Allen said.

"He didn't want to be found. He wanted to run away from his life," a family friend said.

It is understood Matthew's family never gave up hope of finding him alive, believing he had run away to get some "time out".

Originally published as How Matt survived 9-week nightmare