Police officers who searched for the children in the freezing and dirty water told a press conference today they would never forget the face of Tayne's sister when they pulled her out alive.
The siblings were in the vehicle with their dad, who had gone to his worksite at an Adairs Road goldmine earlier in the evening to refuel a pump being used to drain the pit.
About 7pm, on their way home, the vehicle plunged over a bank and landed upside down in the water. The father scrambled out in the dark and ran about 3km for help.
Sergeant Russell Glue of Hokitika police, who was first on the scene, said he initially believed both children had died.
"All I saw were the wheels of a 4x4 submerged upside down in a pond, and my initial thoughts were that no one would have survived," Mr Glue said.
He and senior sergeant Allyson Ealam, the officer in charge of the Greymouth police station, waded waist-deep into the water. Mr Glue smashed the back window and then realised someone was alive.
A tearful Mrs Ealam told the press conference she heard a small voice call for help.
"Russell reached in and pulled the girl out. He handed her to me and I passed her to another staff member. I will never forget that little girl's face."
Having rescued the girl, they expected her brother to follow her out but he did not.
"We fished around for him. We could feel him but we could not get him," Mrs Ealam said.
The vehicle had to be pulled from the pond before police could recover Tayne's body.
His sister was suffering from hypothermia and shock and was taken by rescue helicopter to hospital.
"It was bittersweet for the family. We wanted to return two children to the parents, but that did not happen," Mrs Ealam said.
West Coast police area commander Inspector John Canning said they did not know at this stage how the vehicle ended up in the pond.
"We will not be speculating until we have spoken to the father later on today."
Mr Canning said the family, who arrived at the scene soon after the girl had been pulled to safety, were "initially elated".
"However, the happiness waned when they learned their boy had not survived."
The children attended Hokitika Primary School, where it is understood their grandmother was a long-standing staff member.
"We are a close team and the school community have come together," acting principal Jill Cogger and board of trustees chairwoman Sonja Worthington said in a joint statement.
"This is a difficult time for our school and the welfare of our students is our primary focus."
The school had called in an Education Ministry trauma team to help staff and students.
Westland Mayor Maureen Pugh said it was likely many in the small South Island community would be affected by Tayne's death.
"I know the grandfather of the little boy that died, and I know that they'll be hurting.
"It does hit a small community very hard [and] because we are so small there'll be a lot of people connected to the family.