The boulders pockmarked State Highway 6 with craters up to a metre deep, closing the highway for three hours on Saturday morning.
"If one of those rocks had hit a car ... anything short of a truck would have been wiped out. They would have pretty much taken out anything in their path," Constable Mark Pettigrew, of Queenstown, said.
A motorist alerted police and contractor Downer Construction was called - closing both lanes to begin repair work while checking the bluff to ensure there would be no further rockfall.
Const Pettigrew said the two biggest craters were about "a metre deep and 2m long".
Traffic was backed up, particularly on the Queenstown side of the cordon, and it was lucky the volume of vehicles was low at that time of the morning, he said.
Devil's Staircase has a decades-long history of instability and threatening the highway beneath, since the first goldfields access road was constructed in 1866.
Major slips closed the road in the 1940s and 1970s and on September 17, 2000, a massive fall buried the highway at the bluff, with several motorists narrowly avoiding being crushed.
That fall was caught on video and showed a volume of 10,000cu m for the main fall with a dust cloud that could reportedly be seen from 5km away.
Transit New Zealand conducted stabilisation drilling and blasting at the bluff twice in 2006 and again in 2007.
