The SDR is now fully open.
Carl Slocombe, from Caerleon, was travelling alongside the lorry in his car when the incident happened.
He told the Argus that as he approached the bridge he spotted that a crane on the back of the lorry, between the cab and a storage container it was carrying, was sticking up and it was this that hit the side of the bridge.
"It just flipped the front of the lorry up and the whole thing went over on its side," said Mr Slocombe, who was on a shopping trip at the time.
"I'd seen the crane up just before it happened and everything just seemed to go in slow motion.
"I parked up and ran back, and thankfully other drivers had stopped but the men in the lorry had managed to get out.
"It was very lucky for them, and a good job no-one was on the pavement because the lorry had gone through the barrier."
A resident of Spytty Lane, on the Morrisons side of the dual carriageway, said that when the lorry struck the bridge, such was the impact that her bungalow "shook."
The road was fully closed in both directions for a time, but at 11:30am police re-opened the eastbound carriageway.
The westbound carriageway of the road remains closed however, with traffic being diverted, whilst the vehicle is recovered and checks are carried out on the bridge structure.
The lorry on its side on the SDR. Pics: Mark Lewis
Congestion on the SDR:
A LORRY has overturned on the westbound side of the Southern Distributor Road, in Newport, near Carcraft, after hitting a railway bridge.
South Wales Fire Service report that the incident happened at around 10.30am this morning and the two occupants of the lorry escaped before the emergency services arrived.
They have minor injuries and have received first aid at the scene.
Police are at the scene and are diverting traffic. It is understood that train services using the line carried by the bridge are being stopped while checks are carried out.