Lavinia Langi, 43, and her 15-year-old son Lesili Langi were killed when their car left the road and rolled into a creek in Taylorville, near Greymouth.
Police said a 3-year-old boy who was also in the car could easily have been swept away by the fast-flowing water had it not been for the heroic actions of members of the public.
Family member Taina Kini said Mrs Langi and her husband, Aho, had been married for about 16 years and had seven children together.
"It's only the three older ones that seem to know what's going on and the three youngest don't really know what's happening," he said.
"I don't know how to tell them, how to approach them with this kind of situation."
The Tongan family moved to Greymouth in 2007 for the promise of work and a good education for the children.
Mr Kini said Mrs Langi had dedicated her life to her children and husband.
"It's a family that you see there's so much love at home. We find it hard to believe. She's the kind of woman that really loved her husband and children."
Lesili was a "very good boy" who loved his family and his West Coast home.
"He was a character. He loved to make his friends laugh and he's really a smart boy, too."
The family had been overwhelmed by support since the tragedy.
"It's been unbelievable. The mayor even stepped up and did his part, and the families are all over here to support."
Mr Kini said the members of the public who pulled the toddler from the wrecked car had saved his life.
"If it wasn't for them then that 3-year-old might have ended up like the other two," he said.
"Whoever helped we want to thank them for saving his life."
- Matthew Theunissen of APNZ
