There are so many rivers and streams in New Zealand that it is common for people to end up in a river when they are not prepared for it. Unintentional immersion in rivers accounts for over half of the river drownings each year.
Alcohol often plays a big part in these kinds of incidents. Alcohol makes people far less capable of handling the cold and less able to take actions to survive.
In New Zealand there are many roads that run beside and over rivers so motor vehicle accidents are also a big cause of people ending up in rivers. Sometimes vehicles crash off the road into the river but the river can also rise during floods plucking vehicles from the road itself.
The best way to survive is not to fall in at all.