Guests reported seeing two men jumping on seats and a table on the small corner balcony before the fall, said Concierge Kunal Patel.
"The roof he landed on was all bent with the impact. Everyone was really shocked.''
The man was transferred to Auckland City Hospital where he remained in a critical condition, said a spokesman.
Police said the incident was not believed to be suspicious but it was being investigated.
The young man fell from the 16th floor of the All Seasons hotel in downtown Auckland about 10.45pm on Friday, police said.
Guests staying at surrounding hotels looked on in horror as he plummeted about 42m onto a corrugated iron roof.
Firefighters used an aerial appliance to recover the man from the roof as late-night revellers looked on.
A firefighter who attended the scene said the man had "extremely" serious head injuries and was unconscious throughout.
"He hit the roof really hard and broke a rafter. I have never seen anyone survive a fall from that height."
Kunal Patel, a concierge at the nearby Chifley Suites apartments, said he was told of the accident and rushed to one of the Chifley's top floors to see what had happened.
"I couldn't believe anyone could survive a fall from that height. It must have been the corrugated [iron] roof that saved the guy because if he landed on concrete he would have been gone in an instant."
Other guests reported two men were jumping on seats and a table on the small corner balcony before the fall, Patel added.
"The roof he landed on was all bent with the impact. Everyone was really shocked."
The hotel's manager Caz Vance said the man hadn't been staying at the All Seasons but had been visiting a guest.
"It was obviously an extremely serious situation and the emergency services were called immediately," Vance added. "The man was on the 16th floor when the incident happened and he fell about 12 floors onto the roof next door. It was terrible."
A worker at a convenience store next to the All Seasons said the street was packed with club goers but he still heard a loud thud as the man hit the neighbouring roof.
"The next thing there was flashing lights and emergency people everywhere," the shopkeeper said. "No one had any idea what had happened because it happened up at roof level.
"The next thing I saw was a guy being brought down in a stretcher and he had an oxygen mask on. He looked to be in a very bad way."
A spokeswoman for Auckland police said: "We can confirm that a 25-year-old man was taken to hospital with very serious injuries. Early indications were that it wasn't suspicious but the incident will be investigated."
