Judge Brian Callaghan called the June 9 theft "silly behaviour".
He told Milner that the 10-week spell he'd spent in prison should've given him a "taste of what it was like", and asked him if he realised he'd be back behind bars if caught reoffending.
"Yeah, fully," Milner replied from the dock.
Judge Callaghan said that while the charges were serious, the "purposes and principals" of the case could be met by a sentence of home detention.
"The court has to try a lot harder with younger people to avoid them getting onto the roundabout of offending," he said.
Defence counsel Kirsty May said her client's life "turned to custard" when he assaulted his girlfriend last July.
He started drinking to excess, and after a Christmas Eve party last year, he stole a digger.
Milner, of Christchurch's Avonside suburb, drove it around city streets, hurling obscenities at people, before he rolled it on a tight bend. Then after being caught driving while his licence was suspended, he ran away from police officers and when cornered, climbed a tree.
From there, he spat at two constables and threw a stick at them before falling out and being arrested.
Today he was sentenced on charges of assault, dangerous driving, wilful damage, theft, threatening to kill, and breaches of community work and supervision orders.
Ms May said alcohol was behind all of her client's offending but had lately "changed his attitude".
Judge Callaghan agreed, handing out the six-month home detention sentence, as well as ordering psychological reports and treatment for alcohol and drug issues.
Rachel Nicolson-Hitt, St John's acting district operations manager for Canterbury, said Milner's crime was "extremely disappointing" given the backing the service had from the community.
"This person's actions potentially endangered the lives of patients, and it was an insult to the people of Christchurch who so willingly support us and our work," she said.
Garth McVicar of Sensible Sentencing said home detention was never meant for Milner's high level of offending, and police should appeal the "pathetic" sentence.