The Ashburton River has been rising rapidly and flooded some farmland, as heavy rain swells Canterbury waterways, while a holiday park has been evacuated in Omarama.

Strong winds and rain battered much of the South Island from yesterday, causing power cuts in Canterbury and flooding a holiday park in Omarama and houses in Otematata.

Mid-Canterbury farmer Rhys Roberts says the Ashburton River had burst its banks flooding his pastures.

The river flow at State Highway 1 reached 350 cubic metres a second by about 9am and Roberts says he's expecting worse flooding to come.

Canterbury Civil Defence Emergency Management is monitoring key rivers and warned the main impact of the rainfall will be seen later today and into the night.

Further south, heavy rain battered parts of the Waitaki District in north Otago, flooding a holiday park in Omarama and several houses in Otematata.

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