Two police officers wading out to an overturned and nearly submerged 4WD were stunned to hear the cries of a eight-year-old girl trapped in an air pocket inside.
The girl had survived about two hours trapped in the frigid water but her nine-year-old brother was strapped into his seat and had drowned in the accident, which happened at a private gold mine south of Hokitika on Sunday.
The nine-year-old boy, originally reported to be 10, was named by police on Monday afternoon as Tayne Luca Bowes.
The Toyota Hilux, driven by the siblings' father who worked at the mine, had rolled into the pond and filled with water.
The father managed to escape but had to run 6km to raise the alarm.
Senior Sergeant Allyson Ealam and Sergeant Russell Glue expected to find both children drowned when they arrived.
They used firefighters' gear to smash the rear window and were stunned to hear the girl's cry.
"We didn't expect to hear that cry. I thought we were hearing things. Fortunately we weren't and it was all on," Snr Sgt Ealam told a press conference in Greymouth on Monday, Fairfax reports.
The girl was blue with cold but had managed to breathe in a small air pocket at the vehicle's rear, which was above water.
"I'll never forget her little face. It was like she had just woken up, she was shaking, wet and gasping," a crying Snr Sgt Ealam said.
Her father returned to the scene a few minutes later and accompanied her to hospital. She has since been discharged after being treated for hypothermia.
Police are yet to interview the father and daughter to find out exactly what happened. The father was too distraught to speak to police on Sunday night.
"We haven't got the full picture of when he got out or whether he tried and failed to rescue his children," police spokeswoman Barbara Dunn told NZ Newswire.
The Ministry of Business, Innovation & Employment is also investigating the death as a workplace accident.