He became "excited" when he went on to say that he wanted to shoot a family in the head and kill another relative "and take his guns".
Frank Finch, 22, appeared in the Christchurch District Court today for sentencing on a charge of threatening to kill or do grievous bodily harm.
At the time the threat was made Finch was a sentenced prisoner and due for release in August.
He pleaded guilty to the charge and the details, which were suppressed until today, can now be reported.
The court heard that on March 26 he was about to attend an education class.
He said the class was "too boring" and when asked what his aspirations were in life, said he wanted to be "New Zealand's first serial killer and get 51 kills".
Finch went on to say he wanted to kill 12 or 13 people and hide their bodies.
He went into details about how he would kill them and "the pleasure he would get from it".
Finch told the guard that he "didn't give a f**k" about members of his family.
"I'm going to shoot [a relative] in his head and kill [a second relative] and take his guns," he said.
The Corrections officer reported the matter to police, "worried he would go through with his plans".
Judge David Saunders said the comments came in "close proximity" to the Christchurch terror attack.
Earlier this year the Herald also revealed that Finch had "applauded the actions" of the alleged Christchurch mosque shooter in a letter to his grandfather Rod Finch days after the killings.
Rod Finch told the Herald that he found the letter disturbing.
He said his grandson also disclosed that he was associated with a white supremacy gang.
Rod Finch was in court today to support the offender, who appeared in the dock with his arm in a sling.
Defence counsel Allister Davis said his client's comments were "immature and silly statements" that he did not mean.
But Judge David Saunders said given the climate following the March 15 attacks, the comments were wholly unacceptable.
"Whether a flippant comment or not, it was a very ill-judged comment to make," he said.
"He said he wanted to be New Zealand's first serial killer… he became excited as he said it… he said he wanted to have 51 kills."
Davis said his client - who admitted he was addicted to drugs and "just can't stop" acknowledged the stupidity of his words.
"He should never have said it and he now appreciates he never should have made such a stupid comment," he told the court.
He said Finch needed a sentence to "address his problems" and keep him out of jail.
He said his client was "only 22" and while he had a raft of previous charges, he had a "lack of violence" in his criminal history.
"His history is littered with dishonesty offending," Davis said.
In 2013 he was convicted of aggravated robbery.
A year later he was convicted of an assault on police.
He also had convictions for careless and reckless driving.
A non-association order had been sought to protect members of Finch's family from him given the threats.
However, Davis said none of them supported an order to keep the offender away from them.
Davis said Rod Finch had been "a tower of strength" and wanted to continue to offer his grandson support and help.
"These are people who are his family, they want to help him," Davis told the court.
"The person that doesn't want to help Mr Finch is Mr Finch."
Finch himself addressed Judge Saunders about his drug use.
He said he had "tried" to complete drug rehabilitation programmes but "couldn't".
When pressed by the judge on why that was, Finch became animated.
"Because I don't want to quit drugs, I enjoy them… I'm being forced to give up," he said.
"In the media's eyes i am a white supremacist piece of shit… and everyone thinks I deserve to be in jail… I probably do… but I want to get out."
Davis sought a sentence of home detention with conditions - but during the hearing Finch launched into a tirade, prompting Judge Saunders to close discussions and impose his sentence.
"Just give me imprisonment mate… I'll go back to prison, I'll go back to my cell and sit there - it's my home… I'm over it, I'm sick of standing here," said an emotional Finch.
"I do one day want to try, and the next day I don't… I don't know why…
"I feel like I'm being picked on by the system… I'm getting long sentences for little tiny things… I'm sick of it."
Judge Saunders said Finch's actions gave him "real concern".
He acknowledged the 22-year-old had a "troubled" youth but had to hand down a sentence that fit his crime.
"Having considered this matter… I assess this as in the region where an 18 month [prison] start point is appropriate for a threat of this kind," he said.
He said Finch had a significant history and uplifted the start point by three months.
He said there was no signs of remorse or other genuine mitigating factors but gave Finch the standard 25 per cent discount for his guilty plea.
"There will be a sentence of 16 months," said Judge Saunders.
He said home detention would not be appropriate.
"One would hope that he continues to mature," he said.
Finch burst into anger and profanity when the sentence was handed down.
"This is bullshit... yous can all get f**ked... I'll do my whole lag," he screamed as he was led out of the courtroom.
