A missing Australian tennis coach seemed "distant and spiritual" before he wandered into the Waikato River in the early hours of Sunday, police say.
Paul Arber, 38, was last seen swimming in the river at 4am on Sunday by a pair of passersby, police revealed this afternoon.
Waikato Field Crime Manager Detective Inspector Karl Thornton said the pair convinced Mr Arber to get out of the water.
"Walking down from the bridge to the river bank the pair have talked a man in shorts out of the water who told them his name was Paul and he was in Hamilton for a tennis tournament.
"The pair offered to take him back to his accommodation but after he has gone with them up to the bridge he changed his mind and walked off, back to the river."
Mr Thornton said Mr Arber's behaviour was out of character.
"There was not any one thing that caused specific concerns but such behaviours present a challenge for searchers trying to map out possible behavioural patterns which could assist us in finding Paul."
He had been described as acting "distant and spiritual" in the hours before his disappearance.
As a result of the new information, the riverbanks would be searched again using a boat and the police Eagle helicopter, Mr Thornton said.
Mr Arber's parents, Sam and Richelle, had arrived from Australia and were "extremely concerned" and worried their son was alone in an unfamiliar city and may need help.
"Paul is described by his parents as a gentle, spiritual person who is harmless towards other people," Mr Thornton said.
"That's why, like any mother Richelle is imploring anyone who may have information on Paul's whereabouts, even if it's just something that seemed out of place, to get in contact with police."
Mr Arber is a director and partner in Trademark Tennis in Melbourne.
Fellow director Asar Nagar told the Herald Sun newspaper he had taken a small group of teenage tennis players to Hamilton for a tournament. "We're just hoping for good news," he said.
Mr Arber's wallet and keys were found on Cook St while his luggage and passport were left in his accommodation.
He is described as a slightly built Caucasian, 179 centimetres tall (5 foot 9 inches) with short, dark hair.
He was wearing a grey tracksuit with a tennis logo on the left breast of the top and a Wilson tennis logo on the left shoulder of the top.
Any information which may help in locating him can be provided to Hamilton police station on 07 858 6200 or Crimestoppers, anonymously, on 0800 555 111.