Frank Stewart McLean warned that Comey, who was dramatically sacked by US President Donald Trump on Tuesday, would leave New Zealand "in a wooden box", Fairfax NZ reported.
Citing court documents, it said McLean also threatened to bomb the US embassy in a post on the Wellington mission's Facebook page, forcing it to be evacuated.
"The FBI director... Will not leave this country alive," said one message.
"This meeting has been infiltrated already, there is (sic) bombs placed around the meeting by someone."
NZME reported that McLean, 32, pleaded guilty in Rotorua District Court to making a false statement.
He told the news organisation that he could be described as a "troll or keyboard warrior" and had been angered by coverage of Comey's visit.
"The headlines just tipped me over the edge. I did say something stupid. It was just pure frustration and stupid," he said.
The Justice Department said McLean would appear in court again on June 7, when he is expected to be sentenced.
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