MetService has issued a severe thunderstorm watch for parts of the east coast of the South Island, including Canterbury, on Monday.
The watch covers Marlborough, Canterbury Plains, Canterbury High Country, North Otago, Central Otago, Dunedin and Clutha.
A few thunderstorms in these areas could be severe between 2pm and 9pm, producing localised downpours of 25 to 40 mm/h.
Rainfall of this intensity can cause surface and/or flash flooding, especially about low-lying areas such as streams, rivers or narrow valleys, and may also lead to slips.
Meanwhile, barbecues might not be in order on Christmas Day or Boxing Day in Canterbury with rain forecast to fall on both holidays.
MetService forecaster Surprise Mhlongo said there should be some fine patches during Christmas morning, but a "short front" from the west arriving later in the day would bring spotty rain until the evening.
From there, the rain would clear up somewhat on Boxing Day, but return in the early afternoon.
"So if you’re planning to be outside most of Christmas and Boxing days, you could be disappointed."
Christchurch is expected to reach a high of 21 deg C on Christmas Day and 19 deg C on Boxing Day.
Ashburton is set to reach a high of 22 deg C on Christmas Day, while Rangiora will get 25 deg C and Rolleston 23 deg C.
The Canterbury region can expect Christmas Day to be mostly cloudy, with showers developing in the afternoon and northeasterlies easing.
Occasional rain and fresh southerlies were forecast for Boxing Day.
