At 12.30am on July 21 the teenagers, who had willingly consumed datura (a plant with hallucinogenic properties), stole Larry the lamb from the back yard of a Dunedin property and took it to Kelly's home.
There, in a back yard, they used a wooden club to beat the one-year-old sheep to death.
''It was beaten to such an extent that pieces of the lamb's skull and brain were located in the defendant's rear yard the following day,'' Judge McElrea said.
Once dead, Larry was put in a wheelbarrow by Kelly and Tobin and taken back to the house from which he was stolen.
The teenagers set the lamb on fire and left him on the footpath leading up to the house, where he was discovered still burning by neighbours.
Two weeks later, on August 9, Kelly and Tobin were in Dovecote Ave in Corstorphine where they heckled a man on a motorbike.
The teenagers presented knives at the man as he rode past about 3pm, after which the man went to his nearby home and got an aluminium baseball bat to protect himself.
Kelly and Tobin confronted the man, took his bat, and used it to beat him before running away when confronted by other men.
"The combination of those two incidents shows a worrying element of thuggery and gratuitous violence which indicates a very sick state of mind in both of these young men," Judge McElrea said.
"One asks what sort of mind we are dealing with here and what sort of criminal conduct will ensue in the future."
Both teenagers were "very immature" and it was not safe nor practical to keep them in the community, Judge McElrea said.
"They have both had the benefit of supportive and rehabilitative sentences in the past and have thumbed their noses at it. The time has come when the law is not going to be made a laughing stock of."
Outside the court Larry's owner Matthew Johnston told the Otago Daily Times he was pleased with the judge's decision.
"Justice has been done," he said.
Mr Johnston read his victim impact statement in the court, explaining how his two-year-old daughter had been particularly attached to Larry.
Since the incident, his wife and daughter had moved back to Wellington, partly because of what happened to the lamb, he said.
"It pretty much ruined my family's Otago dream. It was the straw that broke the camel's back."
Mr Johnston said he had expected to receive an apology from the offenders, but had not.
"They don't seem to be showing any remorse at all. It makes me wonder what else these two are capable of doing," he said.
Tobin's defence counsel, Sasha Dolby, said he wrote a letter of apology and was willing to attend a restorative justice hearing.
She read the letter, which stated: "I still have no understanding of why I would do such a horrific thing, I have pets myself and would never harm them in any way."
Ms Dolby said Tobin's mother, who attended his sentencing, also wrote to the court expressing her concern about his actions.
"It's contrary to all the effort she's made to bring up her son properly," Ms Dolby said.
Tobin was also sentenced for two breaches of community work.
Judge McElrea said Tobin's criminal history comprised violent offending including assault and aggravated robbery, so the lamb incident was ''not entirely out of character''.
Public defender Catherine Ure represented Kelly, who was also sentenced for a breach of community work, two breaches of supervision and a breach of bail.
In addition, he was resentenced for previous offending which included burglary, dishonestly getting into motor vehicles and theft.
Both Ms Ure and Ms Dolby said the teens had been criticised by members of the public for what they did to Larry.
Prosecutor Tim Hambleton revealed the teenagers made comments about the incident on Facebook pages after being charged, and some comments appeared to be in response to the public criticism alluded to.
Tobin wrote: "Keep talking s*** and I'll kill your cats".
Judge McElrea said the teenagers' offending was "abhorrent".
He said they would likely be separated from adult prisoners while in jail, and he hoped they undertook programmes aimed at rehabilitation.
