CRASHED: According to police reports this car may have been involved in a Westpac bank robbery in Pukekohe before it crashed north of the Papakura on ramp.

Two suspected bank robbers and a member of the public are in hospital with serious injuries after a crash on Auckland's Southern Motorway this afternoon.

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District Commander Superintendent John Tims said at a press conference in Auckland this afternoon that the crash occurred after an armed robbery at a bank in Pukekohe.

Two men armed with a hammer entered the Westpac bank at Pukekohe about noon, he said. They demanded, and were handed, money and ran from the bank. They drove away from the scene in a maroon BMW before being spotted by a police patrol at Drury.

Tims said the police patrol chased the car as it drove under the motorway and onto Great South Rd, before eventually heading onto the Southern Motorway in a northerly direction.

The car was clocked travelling about 140kmh before it collided with a 2008 Ford Falcon ute, causing both vehicles to roll.

Two men were arrested at the scene and taken to Middlemore Hospital along with the driver of the Ford. The two offenders sustained head injuries. The driver of the Falcon has an injured neck. All were in a serious condition, Tims said.

"It is not the result that we would like. We are here to keep everyone safe. We also have an obligation when two people go into a bank rob people of cash and change the lives of people in the bank forever."

The motorway was closed for about two-and-a-half hours after the crash.

Police were in the early stages of their investigation, but were appealing to witnesses, Tims said.

The matter has been referred to the Independent Police Conduct Authority.

Witness Tina Youngs, who was at the Remax office across the road from the bank at the time of the robbery, said a man had emerged from the bank with others chasing him.

"They got down the alley and one man in a brown jacket tackled him," she said.

"The guy took it upon himself to take him down and he was giving him [the alleged robber] a good bashing.

"He was very brave. I was impressed. It was more than what I would have done.

"There were so many people standing around it was hard to see, but somehow the little man [the robber] got away." Jo Wardle said they initially thought the incident was a fight.

"She [Tina] said to me, 'There is a fight, call the cops'. We could see people running and she said 'He is kicking the s ... out of him, call the cops'." "I dialled 111 and they said to me, 'We are aware of the situation. Stay inside'.

"We kind of worked out it was to do with the bank, then we worked out the bank was robbed." The motorway was closed northbound from Drury, for about two hours, before reopening about 3pm, the NZ Transport Agency said.