In what he described as a "mindless act" which could have had serious consequences, the two men in a blue-and-white runabout were spotted by a passing water taxi driver apparently trying to detach the $1000 light from a buoy.
It was floating near the coastline by the Kelvin Heights golf course, in what is known as the "narrows" of the lake.
Mr Black said the water taxi's appearance, about 1pm last Thursday, seemed to be enough to disturb the men, who sped off.
When the beacon was inspected later, only one bolt of three remained in place.
"They were very close to getting the light off," he told the Queenstown Times yesterday.
"We've had to vandal-proof them now."
In a statement, Mr Black said the new safety beacons had been installed to protect unsuspecting "boaties" day and night.
"The consequences of this kind of mindless act could be extremely serious . . . The stupidity of some people constantly amazes me," he said.
Interfering with any navigational sign, be it a buoy or a beacon, was a serious offence. Anyone with any information about the two men should contact him, he said.
