U-TURN: Passengers were alarmed to realise their route had changed

A bus had to be backed off the Auckland Harbour bridge by police after the driver went the wrong way up an off-ramp.

Passengers called the move "insane" and "terrifying", with one describing the event on Twitter.

"So, our bus driver just did a u-turn on the harbour bridge and started driving into traffic," wrote Amanda Preston, from her account @amandapreston.

"...Everyone jumped up and started yelling at him to stop the bus and called the police. Some guys got off the bus."

"Truck stopped in front of us so he couldn't keep driving. Police arrived, backed the bus off the bridge, dropped us in the city."

"[He] didn't seem at all aware that he was doing something insane."

The bus had been travelling from the Shore to the city. It went to take the Shelley Beach offramp, then did a u-turn and headed back north over the bridge in the lanes reserved for south-bound traffic.

Emergency services were called about 1pm.

Inspector Andy Brill from the Auckland police communications centre said police "shepherded" the bus from the bridge and organised a replacement bus for passengers.

"They got it under control fairly quickly. It's definitely not something you see every day."

Preston said after the police got off the bus, the driver tried to jump back in the seat, ready to continue his route.

He was not allowed back on the bus, however.

The bus company's owner, Andrew Ritchie, said he was just glad no-one had been hurt.

The driver had been taken to hospital for a check-up.

"It's unbelievable really, totally out of character for him," Ritchie said.

"We need to make sure he's OK before we can say anything else."

Auckland Harbour Bridge is the busiest in the country, with eight lanes and a "tidal flow" traffic system. More than 170,00 vehicles crossing it each day.