The self-taught artist Tim Hamilton, 20, filmed real 1m long fin eels on a "red screen" in his mother's bath, and his local Manawatu stream.
He then doctored the footage and enlarged the slithering fish to epic proportions.
The videos were then posted online, where they have attracted thousands of views.
The first video, posted last week, shows his brother Raymond pointing out a massive "human sized" eel supposedly in the Manawatu river.
It has now received more than 47,000 views on YouTube, and attracted the interest of an American TV show.
"They were excited in showing it, but they thought it was totally real. They never bothered asking me if it was fake or not," said Mr Hamilton, of Palmerston North.
That video was filmed in his unsuspecting mother's bath, which was lined with red plastic "because you can't green-screen an eel, because an eel is already green..."
"They're very powerful though," he said. "One of them escaped and was trying to bite my other brother's toes and there was slime everywhere."
The second video shows another brother, Joash feeding the freakishly-large fake eel "pizza bread" - which was actually luncheon meat - at a river's edge.
"Joash has a kind of farm of eels that he feeds by chucking some meat in the local stream so we thought we could have a bit of a play," said Mr Hamilton, who also has his own film production company.
"For the shot where Joash is feeding it pizza bread, we put red plastic under the water in the stream. We needed the current to look like it was swimming."
He was delighted the many viewers believed what they were seeing was real.
However, he had to come clean when his local council phoned him this morning.
"The Manawatu River is ridiculously polluted - you couldn't swim in it - and they wanted to know if there were actually eels in there, because it would give them means to get funding and clean it up," Mr Hamilton said.
"I really didn't know what to say. It was getting to be too big a lie."
