All 15 people have been rescued from a bus that crashed into the trouble-plagued Montague Street overpass in South Melbourne, authorities say.

The Metropolitan Fire Brigade said everyone, including the driver, had been removed from the bus after they became trapped in the aftermath of the accident.

The crash happened just after 10:30am.

Fire crews, police and ambulance are at the scene.

A number of those pulled from the bus were attended to by paramedics nearby, including the bus driver, and some were stretchered to hospital.

It took less than an hour to rescue everyone on board the bus. ( Twitter: Metropolitan Fire Brigade )

Commander Andrew O'Connell from the MFB said most of the injuries were minor.

"Mainly cuts and abrasions and bruising, nothing more than that, although, they've been transported to hospital," he said.

"Four were fairly extensively trapped but thanks to the really good work of the rescue crews and the Ambulance Victoria paramedics, they've managed to extricate all those people in less than one hour.

"I think it's a bit of good fortune, but also the height of the rail bridge has been just above their head-height, which has significantly contributed to a good outcome."

Police said the exact cause of the crash was not yet known. ( ABC News: Simon Winter )

Peter Burns was one of the first on the scene.

"I was about three cars behind and I saw the bus just hit it. I just couldn't believe it. I stopped straight away and ran up and tried to help," he said.

"The door was busted, no-one could get in or out. One of the windows had popped so I grabbed my ladders off the car, put them there and a couple people got in to try and help them."

The road has been closed and people are being urged to avoid the area.

The bus was towed away, with tape over the bus company's name, Gold Bus Ballarat.

Gold Bus Ballarat director Matthew Baird said the practice was "standard protocol".

"Whenever there's a vehicle involved in an accident, notwithstanding the severity of the situation, we're out to protect our brand and make sure that we don't tarnish our brand with a vehicle that is obviously in quite bad disrepair," he said.

"That's just our standard protocol for any incident like that where there's severe damage to a vehicle."

Trouble-plagued Montague St bridge under spotlight

Authorities said more than a dozen people had to be removed from the bus. ( Twitter: Metropolitan Fire Brigade )

There have been a number of crashes at the bridge, mostly involving trucks, leading to calls to try and fix the problem.

Nanin Wijesundaru works nearby and said a bus or truck crashes into the bridge at least once a week.

"It was last Friday, around about this time, a caravan went through with a family, their holiday was ended," he said.

Silvio Parisi also works nearby, and said many drivers are just careless.

"I've worked here for 12 years, and virtually every week we see one or two accidents with this bridge," he said.

Inspector Jo Dolan from Victoria Police said it was too early to say what caused the crash.

"There's been a lot of issues with this bridge. It's probably a funding issue to raise it. It's got signs and lights, I don't really know what more you can do," she said.

"I have seen many crashes in 30 years. This is unique. We're just lucky that no-one is seriously injured."

Mr Burns said crashes were happening at the overpass "all the time".

"This time it's bad because it's a bus involved, usually its just trucks but it's an ongoing thing," he said.

"The Government needs to do something about it. I'm a truck driver too. There's [signs], flashing lights and that. I don't know what else they can do but they need to do something."