An online book of condolences has been opened after the tragic deaths of A Rocha's co-founder and CEO.

Miranda Harris, who founded the Christian environmental charity in 1983 with her husband Peter, was killed in a horrific car accident in South Africa on Monday.

Also killed were A Rocha International CEO Chris Naylor and his wife Susanna.

They died when their car collided with another vehicle and plunged over the Swartkops Bridge in Port Elizabeth.

Peter Harris and the driver of the vehicle both survived and are in a stable condition in hospital.

A Rocha has opened a book of condolences for people to share their memories and condolences.

In an update to Twitter, A Rocha said there had been an "outpouring of love" following the devastating news.

"Thank you for your outpouring of love, prayers and words of support following this tragic loss," the charity said.

"We have created a page for you to share your memories, condolences or other reflections. These will be relayed to the Harris and Naylor families."

Joy Mallouh, of A Rocha Lebanon, where the Naylors worked for many years, left a touching tribute in which he thanked them for their service and legacy.

"Chris was in love with nature, because he loved God very deeply. His insight and advice kept guiding us as Arocha Lebanon board, even after he and his family left to the UK," he said.

"We were in continuous correspondence to keep the good work that he and Susanna started in our land that has been suffering from wars for the past 45 years.

"Susanna was Arocha's accountant all through the years, and God blessed their ministry with funds that were donated to build the eco-restaurant in Aana."

He continued: "Chris and Susanna, you have left a great legacy behind you and you will always be remembered for what you were, and your dedication to serve our country through your example in daily practice.

"As for you my dear Naylors, we at Arocha Lebanon are praying for you to receive continuous comfort from the Lord. We love you very much and we pray you would visit us in Lebanon in the near future."