A mother and a son of a Tongan family have been killed following a car crash Monday 16 shortly before 11am while a 4 year old  was taken to Grey Base Hospital.

Lavinia Langi a.k.a Manui Kitekei’aho Langi 43, of Houma, Tongatapu  but  lived in  Greymouth, New Zealand  and her  eldest son, the 15-year-old  Lesili Langi were killed when their car went over a bank and landed upside down in a creek near Taylorville, police said.

The mother's husband Tau’ataina Langi, who worked in forestry, had been on a job in Invercargill when he received the devastating news yesterday afternoon.

Relatives said he returned from where he works and is now with the rest of the family.

The couple have seven children and according to relatives the family migrated to New Zealand in 2006.

A relative said Mrs Langi and her sons – drove in the rain to Taylorville, where they went every week "for the cheap meat".

On the way back about 11am, the car flipped and landed on its roof in a creek beside the road.

The mother, 43, and teenage son – a Greymouth High School student, died, but the three-year-old clambered free from the wreck, likely saved by his car seat.

''When witnesses got there, he was already climbing out of the van,'' a family friend said. 

Another relative said the 4 year old 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.

The boy was taken to Greymouth Hospital with only minor injuries and was discharged this morning.

Police investigation continues.