Fears are growing for a man believed to be tramping near the Rimutaka Incline.
He has not been seen for days.
A car belonging to Timo Aason, a 48-year-old Canadian, was found near the Incline track near Cross Creek on Monday and police say he may be tramping in the area.
Senior Sergeant Mike Sutton said the red Suzuki Swift was parked near the start of the track.
"Weather during this time has been cold, which has given police concern for his safety due to a lack of contact with anyone known to him during this time. It is possible other trampers may have seen or spoken with him since May 30 and if so, we ask that they contact the police."
A farmer had alerted police on Thursday after noticing the car had not been moved since Monday, Mr Sutton said.
Temperatures had dropped below zero overnight Friday and police had deployed a Search and Rescue team to the area to begin an initial search.
"He could have received an injury or had an accident, or he could just be chilling out, but it's a bit of a worry."
Mr Aason was known to be a "bit of a loner" and it was possible he was seeking some time alone, Mr Sutton said.
"We are pretty confident that he's 'gone bush' but we are just a bit worried that he hasn't been seen for a few days."
The search would be focused on the area around Mr Aason's car, Mr Sutton said.
Police expect to scale this down overnight unless further information is received
Mr Aason had lived in New Zealand for years, and was familiar with the district, but was based in Christchurch.
Police were still trying to contact Mr Aason's family.