An 18-year-old reported missing in Sydney's north has been rescued from bushland more than two months after he disappeared.

Matthew Allen was reported missing by his family when he did not return to his home at Westleigh, near Cherrybrook, in late November.

They feared the worst because he had not accessed his bank accounts or taken his phone.

Extensive searches by police and emergency workers had failed to find him.

But on Saturday, two bushwalkers discovered Mr Allen lying in dense bushland only a kilometre away off Elouera Road.

Police say he was extremely disorientated, covered in leeches and bugs and had lost about half of his bodyweight.

He was also believed to suffering from gangrene to his feet and lower legs.

The teenager was so exhausted and dehydrated, he had to be winched out by a rescue helicopter and taken to Hornsby Hospital.

Detective Acting Inspector Glyn Baker says his family had feared the worst.

"His family are ecstatic that he's alive and that he's well," he said.

"He's been reunited with them and they've been to Hornsby Hospital to see him."

Mr Allen told his rescuers he had survived only on water from a creek bed that was almost dry.

Detectives believe there is more to the story but have been unable to speak to him.

The teenager survived Sydney's record heatwave when temperatures peaked above 45 degrees.