Douglas Haywood, Ross Kelly and Mr Kelly's wife Lyndal were among an estimated 19 people aboard a vintage aircraft that faltered soon after takeoff from Pretoria's Wonderboom airport Tuesday afternoon, before crashing into a small factory. 

A factory worker and one of the plane's passengers, South African flight engineer Chris Barnard, were killed. 

On Friday the Albion Park-based Historical Aircraft Restoration Society (HARS) advised Mr and Mrs Kelly were in a critical but stable condition; Mr Haywood's condition was described as critical.

The Mercury understands Sydney grandfather Mr Kelly is the most seriously injured of the three. 

His son, who is a doctor, is among family members who have arrived at his Johannesburg Hospital bedside. 

Mr Haywood, also of Sydney, is believed to have suffered a very serious eye injury. 

News of the crash has devastated the volunteer community at HARS, which works with similarly aged aircraft, including one near-identical to the Convair CV-340 involved in Tuesday's fatal crash.  

HARS president Bob De La Hunty said Mr Kelly and Mr Haywood flew a Convair to Albion Park almost two years ago and as a result were sought out by the operators of a the Dutch aviation museum, Aviodrome, to deliver the the 64-year-old CV-340 

Tuesday's short flight from Wonderboom Airport to Pilanesberg is believed to have been a weight test before the aircraft made the trip to its final home at the Aviodrome. 

"They [Mr Kelly and Mr Haywood] were both on leave, we didn't know what they were actually doing," Mr De La Hunty told the Mercury.

"It was a huge shock to us when we heard what had happened. 

"They're wonderful, close friends and exceptional pilots, greatly experienced not only on airlines but vintage aircraft as well, for many, many years. They have a big passion for saving history and a great interest in all technical aspects, so they are wonderful people." 

It is unclear whether either Mr Kelly or Mr Haywood was piloting the aircraft when it crashed. 

Both men had flown for Qantas for more than 30 years, including as A380 captains, and had a combined 37,000 hours of flying experience, according to Qantas. 

HARS issued the following statement a short time ago: 

"All of us at HARS are saddened by the whole event and our thoughts go out to not only our members' families but all of those involved in South Africa." 

"HARS had no involvement in this aircraft delivery and we have very little information to share other than what has been in the news, and we are respecting the families of our members in not releasing any details that cannot be confirmed directly by the families. 

"Information from family sources today can change at any time, but it is understood that whilst still critical, Ross and Lyndal Kelly are stable, and Doug Haywood's condition is still critical, but until family travelling now arrive, it is unclear how he is doing."