"They pulled out of a service station and on to a connecting road when Mr Simpson's car sped up close behind them." Justice Ian Gault said in the new High Court judgment.
The couple moved into the right-hand lane so Simpson could pass them.
"Annoyed at Mr Simpson's driving, the adult male victim showed his middle finger to them and yelled 'f*** you' as Mr Simpson pulled into a service station on Roscommon Rd," the judge added.
In response, Simpson pointed his hands at the victims and made gun gestures. His brother got a shotgun from the back seat and assembled it.
The couple with children drove away and got on the Southwestern Motorway (SH20), heading north.
Simpson and his brother changed seats and Simpson retrieved a guitar bag with another shotgun and ammunition.
Justice Gault said Simpson and his brother caught up to the victims about 10 minutes later, yelling as Simpson waved around the shotgun.
"He then leaned out of the front passenger window of the car with the shotgun all the way out of the window and pointed it at the adult victims."
He fired a shot, striking the front bumper of the victims' car.
Police later found Simpson, and 39 live shotgun rounds, a 9mm pistol round, and 10 spent shotgun cartridges in his room.
Police found two shotguns that had been in the car, and clothing Simpson had worn, at Whatipu Beach on Auckland's west coast.
Simpson was 21 at the time and his brother was 24.
Simpson pleaded guilty to intentional damage with a firearm, unlawful possession of a firearm, and unlawful possession of explosives.
At Manukau District Court, Judge Mina Wharepouri said the shooting posed significant danger to the victims and other road users.
He sentenced Simpson to 19 months' jail but Simpson's brother got home detention.
Simpson appealed, saying his jail sentence was not the least restrictive outcome in the circumstances and he also should have received home detention.
The court heard Simpson bought the shotgun after being victimised in a home invasion.
Simpson claimed the victims' vehicle inappropriately moved into his lane, and said he never meant to pull the trigger, but only planned to scare the victims.
Simpson's lawyer Ina Stewart said the district court gave excessive weight to the fact Simpson wielded the firearm during the offending.
She also said Judge Wharepouri was wrong to find a home detention sentence would not adequately meet principles of deterring Simpson and denouncing his behaviour.
Stewart also said home detention could remove the harmful impacts of imprisonment on young first-time offenders.
Crown prosecutor Milan Djurich said Judge Wharepouri already considered Simpson's youth and lack of previous convictions, and his personal circumstances.
Justice Gault said Judge Wharepouri did not fail to consider principles other than denunciation and deterrence.
But he said the district court judge gave too much priority, when sentencing the brothers, to the fact Dekota Simpson fired the shot.
"In the circumstances, and having regard to the further evidence on appeal, I consider that was an error," he said.
"The actions of Mr Simpson and his brother put the victims and public in considerable danger, and had significant victim impact," Justice Gault added.
"But the Judge accepted that Mr Simpson did not intend to hurt the victims. The lead charge was commission of intentional damage."
Justice Gault said Simpson's firing of the shot was not the only relevant factor, but there was a marked difference in sentences for the Simpson brothers.
"The further evidence on appeal adds weight to the importance of Mr Simpson's rehabilitation and reintegration, and of avoiding imprisonment as far as possible."
He said 19 months' imprisonment was manifestly excessive.
"A sentence of home detention also better reflects parity between Mr Simpson and his brother."
The jail term was quashed and replaced with a sentence of seven months' home detention.
