Adrian Banciu, 33, was last seen at his home at Macquarie Park, in the city's north, on November 12.
Five days later, after he had been reported missing, Mr Banciu contacted his family and emergency services saying he was lost in bushland close to Berowra Valley National Park, on Sydney's northern outskirts.
Multiple police rescue units, as well as the SES and Rural Fire Service, began scouring bushland near Mt Kuring-Gai to try to find him.
Police believe Mr Banciu was near an aquatic centre at Hornsby on the day after he was last spotted, and he then entered some bush off Galston Road at Hornsby Heights.
Extensive searches have taken place but he has not been found.
Emergency services are scouring bushland for a missing Sydney man. (HANDOUT/NSW POLICE)
Chief Inspector Jason Donald said a large-scale land, water and air search is ongoing but officers worried Mr Banciu was not carrying appropriate supplies.
"He's not equipped for this. He's not carrying a backpack or anything like that, so it's not a planned expedition," he told reporters on Thursday.
"We think he's gone into the bush for a short time and then become lost."
Mr Donald described the terrain as rugged.
"It's all steep valleys and ridges and rivers. It's all bushland, so it's very easy to become lost".
Police and his family have serious concerns for his welfare.
Mr Banciu is described as being of Caucasian appearance, about 175cm tall with a thin build and a black beard.
He was last seen wearing a maroon flannelette shirt, tan shorts and black velcro sneakers.
Police have called for anyone with information about his location to contact authorities.
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