A bottle of hydrochloric acid was stolen from the back of a pool supplies van and thrown on a pregnant woman and her partner in a horrific attack in an Adelaide suburb today.

Witnesses said the woman, 19, and her partner, 28, had walked from the Centrelink office in Oaklands Park and became involved in an argument with a third person - a male described as aged 16 or 17 - at about 1.50pm.

It is understood the six-months pregnant woman and/or her partner started throwing blocks of concrete at the other male, Adelaide Now reported.

He retaliated by stealing a bottle of hydrochloric acid from the back of the van, which was parked in the same street. The driver of the van, who was also leaving the Centrelink office, happened to recognize the male who was having concrete thrown at him and offered to help drive him from the scene.

"But before I knew it, the acid was taken from the van," the driver, who did not wish to be named, said.

"How much acid the woman had poured on her I'm not sure, it all happened so quickly."

The woman and her partner, from Sturt, were taken by ambulance to Flinders Medical Centre suffering from acid burns.

The woman had burns to her right arm.

Their immediate condition is not known but the injuries are not life-threatening, police said.

A 13-year-old witness described the man who fled from the scene as wearing black shorts and running through the car park of the nearby Bunnings store.

"I could hear all of this screaming and yelling coming from the car park and then people were running everywhere to help this woman," he said.

"There was a lot of panic and before I knew it there were two police cars and two ambulances." The offender, who is known to the victims, has not yet been caught.