Three armed robbers and a getaway driver are on the run after a brazen daytime ram raid on a bank in Sydney's CBD.

A black Porsche four-wheel drive crashed into the front of the Westpac branch on Kent Street about 11:15am (AEST).

Police say the Porsche crashed into a wall beside the ATM and three men armed with sledgehammers entered the bank and took money from the tellers.

The thieves then escaped in a blue Subaru sedan that was waiting for them. Police say both vehicles were stolen.

Witnesses say the black four-wheel drive crashed into the bank, then reversed and drove into the bank again. ( Twitter: Matthew Harper )

Police say they have fielded a large number of calls from witnesses who described the incident as a ram raid or armed robbery.

However, Acting Superintendant David Roptell says police have obtained CCTV footage from the scene and appealed for any additional witnesses in the area at the time of the incident to come forward.

"We're still talking to witnesses and talking to anyone that was in [the bank], we're still appealing for people that were in the area at the time," he said.

"And we're just going through the premises to find out if we can find any evidence.

"It's very important that any witnesses in the area that may not have been here at the time to talk to us, but they may have some footage on their phones and any other information that may assist us with our inquiries [come forward]."

Witnesses 'shaken up' by daylight bank robbery

Acting Superintendant Roptell says the violent theft of an as yet unknown amount of cash has left staff in shock.

"A couple of them are rightly shaken up from the incident. They're seeking treatment at the moment," he said.

Paramedics are at the scene and have treated a number of people for shock.

Witness Cheryl Hudswell had just walked out of the Westpac branch when the four-wheel drive crashed.

"I just thought it was an accident and I thought this guy was trying to get away," she said.

"But then he - with a lot of force - had the second go at it. I thought 'He's a maniac'.

"The next minute these two guys jumped out with sledgehammers and everything else and started smashing.

"I usually go to that ATM. I always got to that ATM ... normally there's a line-up in this street for that ATM.

"There were people everywhere. It was like the middle of the city, everyone's taking photos of them and that sort of stuff and they just kept going. Nothing was going to stop them."