Detective Inspector Aaron Pascoe said police are building a timeline of Gelberger's movements and the path he took for his swim.
Gelberger is thought to have begun his swim just after 10am from Narrow Neck Beach.
"He is thought to have swum out across Cheltenham Beach, towards North Head, with the intention of coming to shore again at Devonport," Pascoe said.
"At this stage, police believe Mr Gelberger was struck by a vessel as he approached North Head."
After Gelberger was reported missing a search which included an Auckland harbourmaster boat, was launched, said Pascoe.
"At 12.05pm staff onboard the harbourmaster boat came across two men fishing in a small, white, fibreglass dinghy, powered by a Seagull outboard engine.
"The two men were fishing off Takapuna Head, and told the harbourmaster staff that they had seen a swimmer that morning.
"Police need to speak further to the two men in the dinghy about the swimmer that they saw."
Pascoe said the dinghy is not believed to be the vessel that struck Gelberger.
"Several members of the public have already come forward to police with information," he said.
"Police would also like to hear from anyone else who may be able to assist with the investigation."
Anyone with information is asked to contact Auckland City Police on (09) 302 6406.
