A fifth construction worker has died of injuries he sustained when a Stellenbosch building collapsed during renovation work on Monday afternoon, said the department of labour.

Four others were seriously injured in the 2pm collapse, which was triggered when a wall toppled, said spokeswoman Zolisa Sigabi.

Stellenbosch municipal fire brigade chief of operations Tasso Steyn said the two-storey building, located in an industrial park in Onder-Papegaaiberg, used to be a KWV wine cellar, but was being remodelled to form an industrial hive.

"This was one of the last remaining structures which still needed to be renovated," he said.

Four people died when they were trapped underneath a concrete slab. The fifth died of his injuries later in hospital.

Rescue workers could recover only one of the bodies from the rubble before the site became too unsafe, said Steyn. The body had been located close to one of the exits.

"The whole time we were standing there the building was moving," said ER 24 spokesman Nico Grobler, adding that all that was keeping it up were the two outer walls. "It was totally unstable".

He said it had since been decided that there was "absolutely no way" to make the building safe and that it would be best to collapse the whole building and remove the rubble to recover the other three bodies.

One of the 13 people injured in the collapse was pinned at his right elbow between a windowsill and the floor. Another managed to free himself, but lost the fingers on his right hand, said Steyn.

Paramedics took six people to hospital, three to the Stellenbosch provincial hospital and three to the Stellenbosch Medi-Clinic. The other seven sustained minor injuries.

Grobler said all the workers had been trying to get out of the building when it collapsed. "They started hearing noises, cracks and that." The seven slightly injured workers got out before it started coming down.

All the injured were employed by the construction company renovating the building and all were men, said Steyn.

According to Grobler, one of the men taken to the Stellenbosch provincial hospital was going to be transferred to a larger hospital in Cape Town in an attempt to save his right arm which had a "nasty fracture".

The other two were being held mainly for observation. "It seems they've got a couple of minor fractures. Nothing serious," he said.

The two surviving workers at the Medi-Clinic had sustained multiple crush injuries, including fractured arms and head injuries.

The site had been sealed by Monday night and labour investigators were on the scene.

"We are deeply saddened by this tragedy," said Sigaba. "We will do everything in our power to investigate the circumstances around the incident," she said.

Department inspectors would continue gathering evidence and conducting interviews on Tuesday morning and would announce their findings as soon as possible.