He became famous when on Saturday 19 March 1932, he upstaged Lang at the opening of the Sydney Harbour Bridge, before a crowd of 300,000 people. He was not a member of the official party but, on horseback and dressed in his military uniform, he was able to blend in with the escort party of NSW Lancers. Lang was about to cut the ribbon to formally open the bridge, when de Groot rode forward and drew his ceremonial sword, making to cut the ribbon and declare the bridge open "in the name of the decent and respectable people of New South Wales". While many accounts say de Groot succeeded in slashing the ribbon, at least one eyewitness has disputed the claim and suggested it was probably broken by the hooves of his rearing horse.