A Canberra man has helped avert a calamity at the Tour Down Under in Adelaide after an inflatable finish line collapsed, almost taking riders with it.

Ross Dobson was watching the prelude race when the inflatable arch failed and fell onto the track, with riders expected to come through any second.

"I think they were due in probably 20 or 30 seconds' time and the thing collapsed," Mr Dobson said.

"I would say it was a generator failure or something along those lines, and it was falling onto the track and it was easy to see there was going to be some carnage at that particular point on the circuit."

Seeing what was about to unfold, Mr Dobson, who is 187cm tall, jumped the fence onto the track to hold up the sign so the cyclists would be able to continue through the race.

He said a quick-thinking spectator across the track saw what he was doing and followed suit.

"I think there would have been a few injuries, they (the cyclists) were fairly bunched up and it was probably seven or 10 laps from the end of it," Mr Dobson said.

"Then the fun really began because the team cars were following with their bikes [on the roof] ... and we just weren't tall enough for those guys to come through."

Mr Dobson said one of the bikes on top of a team car flipped off the roof, but again thanks to his quick-thinking the bike was spared from any damage.

"I caught that mid air and I whacked that back on top of the rig and then I think enough people had sort of seen what was going on and they started to help dismantle the device."

The inflatable sign was quickly dismantled and taken off the track before the riders came back around in the next lap.

Mr Dobson, an avid rider himself, had flown into Adelaide specifically for the event.