Motorsport New Zealand chairman Brian Budd said drivers must also adhere to other rules and regulations, and stewards were investigating whether the crash was a result of driver error.
It was not known at what speed the car was travelling.
Ian Bayes, who was standing only 40m away when the car slammed into the stock underpass on his farm, said the brothers rounded a corner too fast.
"There's a bit of a hump and it's hard to see around the bend to see how tight it is," he said.
"He held it straight and didn't go into a slide but just couldn't slow up, he was going too fast. He slowly came off the road and came pretty close to bringing it back up, but his front wheel hit the underpass abutment and took all the railing off."
The vehicle made a "hell of a bang" as it slammed into the two-tonne abutment with such force that it dislodged the concrete block from its mounting.
Mr Bayes immediately thought both men must have been killed instantly.
"It stopped so quick from going so fast. They're lucky to get away with their lives, especially the driver, who was on the side that hit the parapet.
"The tubing around the driver was all right, but there was no tubular framing beneath him and there was no chassis in the car ... it just opened up like a can.
"There was a loud crash, bang, and smoke for a start. I was worried the thing was going to catch fire, but the following cars all carried extinguishers.
"By the time I came back out after ringing 111 they had it all under control."
Race organisers were last night in contact with Stuart Scoular's family, who asked that no information on his condition be released.
Mr Budd said such serious crashes did not happen often.
"There are rules and regulations [on] the running of the event and the cars to reduce the risks, but they are running on public roads at a reasonably high speed so it's not a sport that's immune from these sorts of incidents," he said.
"This is getting toward the upper end of the scale, but they do happen."
