Cows are causing concern in the Selwyn District after two separate car crashes with stock in a week.

A woman was taken to Christchurch Hospital with serious injuries this morning after her vehicle collided with a cow, before ploughing through a fence and hitting a water tank.

Last Thursday, former Selwyn Mayor Bill Woods' car hit a cow.

Senior Animal Control officer Steve Clarke says it's more of an issue in winter.

"At this time of the year, if there's less feed around, the stock pushes through fences and that trying to find some fresh grass, so it can be a bit of a concern."

Clarke says it's the farmer's responsibility to keep stock off the road.