The New Zealand farmer turned his motorbike towards the noise and was shocked to find a woman huddled on top of her car, floodwaters swirling around her.

"When I saw Rhoda sitting there on the car, I knew time was certainly against us," he said.

Rhoda Davidson had been trapped on the roof of her car for nearly two hours after it was swallowed by the flood caused by a turbulent Easter storm near Oamaru, south of Christchurch.

The car had been swept off the road and into the water, before becoming caught on a fence. Ms Davidson had broken a window and climbed on to the roof, where she watched as the waters rose beneath her.

Upon reaching the scene, Mr Borst turned back to get his tractor but by the time he had returned to Ms Davidson, the water was flowing too swiftly for the tractor to be of any use.

That's when he had a brainwave. Acting quickly, he swapped his tractor for his digger, which he drove into the surging flood.

"When I took the digger down there, the frightening thing was the unknown - had the road washed away? Could the digger keep going?"

An off-duty policeman, Peter Muldrew, happened to be nearby when he heard the reports of Ms Davidson's predicament.

The search and rescue co-ordinator came across Mr Borst and the digger - then climbed into the digger's bucket.

Mr Borst said his "old hands were shaking" as he manouevred the bucket, complete with Sergeant Muldrew, out towards Davidson, aware he could be swept away at any moment.

When Sergeant Muldrew reached the car, the severity of the situation hit home.

"I remember I got boomed out in the bucket across to the car and we got up to where the A pillar is on the vehicle," he said.

"I looked down and saw the water level was right up to the rear vision mirror of the car. Looking around, I realised it wasn't the safest place in the world to be."

Davidson's car was almost completely submerged when Sergeant Muldrew reached for her and placed her into the bucket, pulling her to safety.

Ms Davidson was treated for mild hypothermia but was otherwise unharmed.

The men's actions "undoubtedly saved her life", the Royal Humane Society said, awarding them for their bravery on Monday, as Ms Davidson looked on.

"It was quite surreal, how it all transpired," Mr Borst said. "I must admit afterwards, on reflection, it was a bit frightening.

"It was fortunate enough we had a favourable outcome in a pretty sticky situation."

Mr Borst's wife was "horrified" when she saw the photos of the rescue but his family were "chuffed" with the award, he said.

Sergeant Muldrew said he was just doing his job.

"Of course my family and that are pretty proud, so it's kind of nice," he said.

stuff.co.nz