The man was discharged without conviction earlier this month.
Asked at his post-cabinet press conference today whether he knew the man's identity, Mr Key replied: "I do, yes. I can't divulge my source, it's not an official one. I asked someone who I thought might know and they told me the name."
A recent newspaper survey showed that most members of the public knew the man's identity, which is reported to have been revealed on the internet.
The man's identity came up during questions about a Law Commission report, released on November 6, which recommended that court proceedings should be more open and rules governing name suppression should be tighter and more transparent.
Justice Minister Simon Power said the Government would have a close look at the recommendations, and he believed there was a good case for tightening the procedures.
Mr Key said today he agreed with Mr Power.
"I also think it's very difficult with name suppression in the modern world because the internet and various other communications devices mean people generally circumvent that process anyway," he said.
NZPA
source: newshub archive
