Watch the horrific moment cars crash head on in Sydney Harbour Bridge carnage, as two die and mum-to-be is rushed to hospital

Horrifying footage has emerged of the head-on collision that claimed the lives of two drivers on the Sydney Harbour Bridge.

Four cars and a bus collided just before 2pm on Thursday with emergency services arriving at the scene to find one man dead among the wreckage.

The male driver of another vehicle - who died at the scene a short time later - had to be cut free from the wreckage by fire crews.

Fortunately, the bus had no passengers on board and the driver was uninjured.

Dashcam footage of a car travelling northbound, exclusively obtained by Daily Mail Australia , captures the entire crash.

The vision shows a blue Hyundai i30 hatchback travelling in the outside lane before suddenly drifting into the oncoming traffic.

For a few horrifying seconds, it didn't alter its course.

It then collided head-on with a silver Mercedes, which spun out of control and smashed into a bus before ended up facing backwards into the oncoming traffic.

The blue hatchback then smashed head-on into a Hyundai Accent hatchback before it was briefly thrust into the air with the impact.

It then spun around 180 degrees and came to a stop.

The entire front section was completely crumpled as smoke billowed out of the destroyed engine.

Superintendent Clayton McDonald told reporters the blue Hyundai had travelled into the oncoming lane for an 'unknown reason'.

'The investigation is ongoing, but the Harbour Bridge is covered by cameras which are recording at all times' he added.

Two other adults, including a pregnant woman, suffered minor injuries and were rushed to Royal North Shore Hospital where they remain in a stable condition.

The two other drivers involved will undergo mandatory testing, police said.

All southbound traffic on the bridge came to a halt and only one northbound lane remained open, causing traffic chaos in the city on Thursday afternoon.

Vision from the Harbour Bridge showed two hatchbacks and an SUV with major damage as well as debris spread across several lanes.

Police are now working to notify the next of kin of both victims.

A motorcyclist was arrested on the bridge after he allegedly breached the road block and drove through the crash scene.

At least three officers were seen pinning the motorcyclist to the ground in vision from the bridge captured by nearby motorists.

'A gentleman who was riding a motorcycle entered the crime scene in an attempt to go past and get across the bridge,' Superintendent Clayton McDonald told reporters.

'That gentleman drove towards police who stood in front of him and were able to stop him.'

The incident left one police officer with 'minor bruising to his leg and some grazes on his hand'. The motorcyclist was taken to Day Street police station.

Superintendent McDonald said initial inquires suggested the motorbike was unregistered and the rider's licence was suspended.

He is expected to be charged with a series of driving offences.

Delays on the city side of the bridge saw traffic backed up as far as the Anzac Bridge and the Western Distributor with queues stretching as far as 12km.

Southbound lanes were closed for over an hour following the crash, causing traffic to back up to just south of the Spit Junction.

Delays could still affect commuters for several hours after two lanes in both directions began to open at 4.15pm.

Transport for NSW Coordinator-General Howard Collins urged motorists to avoid the area and consider alternative routes, including those travelling to Qudos Bank Arena for the Olivia Rodrigo concert tonight.

NSW Police said a crime scene had been established. No arrests have been made.

Investigations are continuing.