His admissions came on the day a four-week trial relating to the crash was scheduled to commence, after the trial charges were downgraded.
A large number of members from his church, which has renamed itself the Plymouth Brethren Christian Church, were present at the sentencing.
James Wearmouth's father Jeff said the sentence was "very fair" and as Christians, his family has forgiven Stewart.
"We Christians understand what forgiveness is. We reminisce James' life but we have to keep moving forward," he said.
Susanna and James Wearmouth died almost immediately after the crash.
Stewart's daughter, Sadie, died shortly after from her injuries.
Judge Keith de Ridder noted that 49 character references for Stewart carried the common theme of compassion, forgiveness and a deep sense of loss.
Along with four months' community detention, Judge de Ridder imposed a curfew upon Stewart between 9.30pm and 5.30am daily for the next four months, and disqualified him from driving for one year.
The day of the crash, Stewart and his family had been on their way home from Baylys Beach with other church members when the vehicle lost control on a moderate right corner and crashed into a tree.
Five passengers were seated inside the five-seater Mitsubishi Triton. Two in the ute's back tray were thrown out and suffered moderate injuries.
Four passengers were unrestrained.
Stewart had been drinking and blew 298 micrograms per litre of breath when breathtested about three hours after the crash.
The legal limit — lowered in 2014 — is 250 micrograms.
Crown lawyer Kyle MacNeil said Stewart's driving fell below the required standards, although he submitted a non-custodial sentence would provide sufficient deterrence and denunciation.
Stewart was represented in court by two lawyers, Anthony Rogers and Ian Bassett.
Mr Rogers said according to two crash experts, two unrestrained passengers would not have been saved by seatbelts while those seated on the tray were lucky not to have suffered serious injuries.
Mr Bassett said there was a significant history of repeated crashes at night on the corner where Stewart's vehicle hit the tree.
In 2009, the New Zealand Transport Agency declared that stretch of road as the second worst black spot in Kaipara while the Kaipara District Council recognised various improvements needed to be made, Mr Bassett said.
Loose gravel reduced friction and grip on tyres, he said.
Judge de Ridder said those factors were present on many roads, especially those in rural Northland, which increased the level of care required while driving.
"It was dark but the weather was fine. It is clear that you have been to this beach many times and therefore you were well familiar with this road.
"The relevant facts then are that you were driving with a significant passenger load, the tyres were underinflated, and it's accepted that it required greater steering power when trying to negotiate this bend," Judge de Ridder said.
