The Crown Solicitor at Whangārei, Richard Annandale, said the dangers of the unlawful possession of firearms manifested itself on July 26 after Hayes' "deliberate" supply of firearms to Patterson.
"Mr Hayes showed total disregard for the restrictions for the access and use of firearms in New Zealand," he said.
"Given that military background, he should have known better."
Orginially from Christchurch, Hayes had returned to New Zealand after working overseas in war zones like Laos for his company Phoenix Clearance, which specialised in removing landmines and unexploded bombs.
He served in the New Zealand Army in the 1980s before also serving in the Australian Army as a combat engineer, which included a deployment to Cambodia to clear minefields.
A letter of support for Hayes was provided to the court by a colonel, whom Hayes had served with in Laos.
Defence counsel Arthur Fairley said Hayes had "tremendous sympathy" for the Campbell family and Pipe, who was in court today for the hearing.
Hayes pleaded guilty to the nine charges against him in May and told police after the shootings he felt "guilty and responsible" for what had happened on Mt Tiger Rd.
Hayes was charged with three counts of unlawful possession of a military-style semi-automatic weapon (MSSA), three charges of supplying an MSSA to an unlicensed person, and three counts of supplying firearms to an unlicensed person.
The firearms he supplied, between March 1 and May 1 last year, included a Mossburg 12-gauge shotgun, Gevarm .22 calibre rifle, AK-47 replica semi-automatic rifle, and Saiga 12-gauge shotgun, and 7.62 calibre rifle with various rounds and magazines.
Judge John McDonald said the weapons, modified by Hayes, were the kind designed to "hunt humans".
"The way you modified those three firearms there could be only one purpose, apart from target shooting, and that was to kill people," the judge said.
"They were rifles used by militaries around the world to kill people."
He had met Patterson in about 2013 or 2014 after the would-be gunman responded to a letter drop by Hayes seeking properties to trap possums.
The pair developed a friendship before, in 2016, Hayes agreed to be a referee and provide a letter of support for Patterson's ultimately unsuccessful application for a firearms licence.
Police rejected Patterson's application, deeming him not a "fit and proper person" to handle guns.
Several times between March 1 and May 1 last year Hayes visited Patterson with his son and sister's partner.
He took with him the military-style weapons and ammunition, including a replica AK-47 and semi-automatic shotgun.
While Hayes held a firearms licence from June 2014 to August 2017, when it was revoked, he didn't hold the special licence required to possess military-style semi-automatic weapons.
It is extremely difficult to get such a category of firearms licence in New Zealand.
At Patterson's home, the pair would use the weapons for target shooting.
Patterson had a "shooting platform" built in his backyard, which was the subject of a police visit prior to the fatal shootings, and neighbours recalled hearing regular gunfire coming from Patterson's home.
TradeMe was also a source of weapons for Patterson, who bought 10 firearms on his account using Hayes' name and firearms licence between September 2016 and July 2017.
The guns included two 7.62 calibre AK-47-type weapons.
On the day of the shootings, Hayes was interviewed by police, before being interviewed a further three times.
He told police he felt "guilty and responsible" for what happened to the Campbells and Pipe, and felt he had contributed to the killings.
Police have completed its investigation into Patterson and the shootings but the findings will not be published until after the coronial process is completed.