A man who jumped from a moving car on Auckland's Harbour Bridge then leapt off the bridge - and survived the fall into the water.

The jumper cheated death in the 43-metre fall and managed to swim to safety. He was taken to hospital with moderate injuries.

A motorist watched stunned as the incident played out on the bridge, which caused lengthy delays for citybound traffic this morning.

His friends in the car said was having a "bad trip" and they were taking him to hospital.

Matt Albert-Wihone told the Herald that he was driving to work around 7.30am when he saw a man jump from a moving vehicle - before sprinting off the bridge and jumping into the water.

"We were on the way to work on the bridge.

"Just before you reach the high point in the bridge, the back door of the car flew open then and he just stepped out like it was stationary.

"This was at 80kmh, he took a few tumbles on the side of the road and before we had time to stop he was off. He was up straight away and without hesitation, he just jumped [off the bridge].

"It was shock and horror. I was in shock and I didn't want to look over."

The Harbour Bridge is 43 metres above the water at its highest point.

The people the man was travelling with had stopped the car when the passenger jumped out.

They had spoken to Albert-Wihone explaining they were taking the man to hospital because he was having a "bad trip", he said.

He could see later the man was resurfacing and he was swimming towards the city.

"I called emergency services straight away. He is lucky."

The Harbour Bridge has a 43.3m clearance at high water, according to Engineering Heritage New Zealand.

Police confirmed they responded to reports of a man who had jumped off the bridge around 7.30am.

The man had managed to make it to the shore nearby and was taken to hospital.

Police had not been seeking the man prior to the incident, a spokeswoman said.

St John said they transferred a man from the Auckland Harbour Bridge to Auckland Hospital in a moderate condition.