A young man purporting to be a pro-commercial whaling supporter named Jay Pryor was interviewed by Paul Henry after being profiled in a Fairfax newspaper story at the weekend, TVNZ said in a statement.
"The man had gone to reasonable lengths to dupe the public by creating a false name and setting up a fake website," the statement said.
"Paul Henry's suspicions were raised during the on-air interview because of the interviewee's lack of knowledge, despite a Breakfast producer having spoken to `Jay Pryor' the night before the interview, and believing he was plausible."
While the interviewee was at TVNZ he had his photograph taken with Prime Minister John Key, who was also on Breakfast this morning.
"Breakfast producers have advised the Prime Minister's Office of this event," the statement said.
"As a result of this prank, Breakfast is reviewing its set-up interview procedures and apologises to viewers for the error."
TVNZ said it believed the interviewee is a television comedian named Guy Williams.
