Passengers trapped in a car swept off the road by floodwaters yesterday were "incredibly lucky" to be saved, Mid-Canterbury Federated Farmers president David Clark says.

Timaru, Ashburton and the Selwyn district all declared civil defence emergencies yesterday, as flooding closed roads and bridges and inundated farms.

The occupants of the car had a "legitimate reason" to be on the road, as they were trying to get to a flight leaving from Christchurch, Clark told The Country's Jamie Mackay.

They were travelling at 4:30am in very heavy rain with poor visibility and thought they would only be dealing with water 200mls deep, he said.

"What they didn't know was that there was a washout in the road ahead of them and that 200mls of water suddenly became water many feet deep and that swept them away."

"They were incredibly lucky in that my neighbour across the river from me, Chris Allen, who is a national board member for Federated Farmers, was out checking stock at half past four in the morning, saw the vehicle drive down the road but then became aware that they hadn't gone around the corner and carried on."

Allen noticed the car's lights in the floodwater "bobbing away from his farm" downstream into the neighbour's property and called 111, Clark said.

While the occupants of the car had also called 111, Allen contacted Clark, as he recognised the fire brigade might not be able to help, due to the extreme conditions.

"Chris had the presence of mind to ring me…because the water was so deep and the car was so far from the roadway that the fire brigade weren't going to be able to do a lot to help - and that it was going to need a big machine to get to them carefully and safely."

Allen "raced over with a tractor and some life jackets" for Clark and the people in the car.

The tractor reached the car, which was trapped in very deep water, Clark said.

"[We] smashed the window of the car and managed to get them to climb out and up into the tractor and back to safety."

The rescue highlighted the danger of floodwater, and although there had been a "good outcome," Clark believed the occupants had had a lucky escape.

"If it hadn't been for the fact that Chris was up checking stock I think we probably would've had a tragedy."