The SA Weather Service has warned that people should stay out of open fields and high areas when there's lightning.

The warning came after three people were killed and two critically injured after being struck by lightning during a prayer session on Sunday afternoon.

The incident occurred during a flash storm in the suburban area of Kensington, Johannesburg, on a steep hill used as a prayer spot by the church.

The group was apparently taking part in a prayer service when lightning hit.

A member of the church, who did not want to be named, said she was still in shock as she also attended services on the hill.

"I'm really not in the right frame of mind to speak out right now. All I can say is that three lives were lost," she said.

She said she was not at the church service at the time but arrived soon after the lightning had struck. "I was called immediately after it happened and I went there."

She said the incident was a wake-up call for people to be cautious during the rainy period.

"This can happen anywhere. It really is important to take caution whether you are at home or on the street."

The injured church members are receiving treatment at Charlotte Maxeke Academic Hospital.

Weather service spokesperson Hannelee Doubell said when lightning strikes it always looks for the highest point.

"You should not hide under a lone tree. The tree is the highest point and the lightning looks for the highest point. If you hear the thunder, you must just go indoors. This means go inside the house and close the windows and doors. Don't just go under the veranda because lightning can strike you there," she said.

Doubell advised that when people hear thunder, they should rush indoors to be safe.

The other dangerous spot when there is lightning is a place with water.

"Lightning likes water. Don't bathe or take a shower. Don't be swimming or on a boat, get out of the water."

Doubell said people should also not be close to wire fences when there was lightning as the iron can transmit the electricity.

She said even when in the house, people must unplug all electrical appliances and switch off the plugs. Windows must be closed as lightning can strike through the window.

"You must not be on a landline because it can definitely conduct the electricity to you. If you are in your car, close the windows. If you are on a motorbike, you have to stop and take shelter because it can be hit."

Those who play golf should not play with umbrellas as the metal on the umbrella can transmit the electricity. The same applies to walking under an umbrella.

"Rather find shelter than continue walking. Lightning always looks for the highest point. The lightning is normally not more than 15 minutes. Rather wait a bit. Rather be safe than sorry," she said.

She also warned against playing soccer or any sport in an open field.