It was carrying a total of seven people, including Mrs Stewart who was the front seat passenger, Sadie, who was in the rear left seat and Jamie in the middle rear.
The Serious Crash Unit is investigating the crash and will look into what charges, if any, may be laid against the driver.
The spot, where the trio died, was the same one where another fatality had occurred almost a decade ago.
Volunteer firefighters working to free those trapped in the vehicle at the time of Monday's accident noticed a cross, marking a previous tragedy, nailed to the tree.
Police later confirmed it was in commemoration of a 39-year-old Dargaville man who'd died in December 1997 following a crash in the same area.
The trio's death on Monday put the road toll at 11, the worst road toll for Queen's Birthday weekend in 27 years.
New Zealand's worst ever Queen's Birthday weekend on the roads was in 1973, when 24 were killed.
NZH