In Milne's arsenal was a Ruger Mini 14 semi-automatic rifle - prohibited unless owners have a special licence endorsement - that he failed to surrender during an amnesty period after the firearms law changes.
Four of the firearms found by police were loaded and another was unsecured.
Milne appeared for sentencing in the Alexandra District Court on Friday after admitting charges of unlawful possession of a prohibited firearm, and possession of 11 non-prohibited firearms without a licence.
The first charge carries a maximum penalty of five years in prison.
Counsel Kieran Tohill said the defendant used the firearms for legal purposes, including pest control and euthanising farm stock, on the three family farms he managed.
He had not been using the prohibited semi-automatic rifle because its magazine had been lost.
"It's not gang-related, not drug-related ... the only thing he's done wrong is own these things.
However, that was a "serious mistake and a misjudgment".
Judge Dominic Flatley said there had been a "huge increase" in the number of illegal firearms in New Zealand and gun crime was of major concern to the police, courts and society at large.
"Firearms are dangerous items, and they need to be controlled in accordance with the law."
Taking account of the defendant's guilty pleas, and his apparent use of the firearms for legal purposes, he came to an end term of 18 months' prison.
That was converted to eight months' home detention.
The firearms and ammunition will be destroyed.
