Lavinia Langi, 43, and Lesili Langi, 15 were killed when their car went over a bank and rolled into a creek as they drove through Taylorville, near Greymouth, shortly before 11am.
Senior Sergeant Allyson Ealam said the toddler, who police said was 3-years-old but who a family member said was 4, had started clambering out of the upturned car and into the fast-flowing creek when he was rescued by members of the public.
Had they not come to his aid the boy could "most certainly'' have been swept away.
"They're our heroes,'' Ms Ealam said.
"I know it's a tragedy that a mum and son have lost their lives. But honestly, we could have lost that little chap as well.''
When police arrived, the toddler told them that his 8-month-old sister had also been in the car so they launched an urgent search of the area.
Fortunately, it was established that the girl was being looked after by another family member.
The vehicle was being inspected and post-mortem examinations would be carried out on the bodies today.
The Tongan community is rallying to support the grieving family as are schools, including Greymouth High School, where Lesili was a Year 11 student. His sister also attends the high school, while three younger siblings attend Karoro School.
"It is obviously very sad for us, especially at this time of the year,'' Karoro School principal Maureen Truman said.
"The community is rallying behind the family. Students have written messages to their classmates and we, as a school, are waiting to hear how to best support them,'' Mrs Truman said.
The local Tongan community had been "fabulous'', as had the larger community: "We have had donations of food dropped off to us to give to the Langis, as well as financial donations''.
Greymouth High School principal Andy England said Lesili was a "very special part of the school''.
"He was a character and a trickster and enjoyed playing pranks on his friends.''
He was also involved in the Tongan dance group and often performed.
"I remember seeing him light up when he was leading the dances - he was really amazing.''
The Langi family moved to Greymouth about seven years ago.
Ms Ealam said the crash occurred following a heavy downpour, after which there were at least four other crashes in the region in a short time-frame.
"We want everybody to adhere to the speed limit, to drive to the conditions and to get where they're going,'' Ms Ealam said.
"If you're going on holiday leave plenty of time, have plenty of breaks and have a safe Christmas.''
