We're still of the belief we're in for another cold snap ... in the second week of July.

Single-digit highs and morning frosts are expected to be replaced by heavy rain and windy conditions in the North Island this week, say weather forecasters.

Phillip Duncan of WeatherWatch said people should prepare for wet and windy conditions as a low from the Tasman moves over the country.

"It is most likely to bring in fairly warm, heavy rain for the North Island.


"The temperatures are going to lift during the days and will jump up over the nights.

"It'll be back to double-digit overnight lows for many places."

He said most North Island areas and the Nelson region will have heavy rain tomorrow.

"We also expect thunderstorms and there may be a chance of rain warnings for parts of the east coast."

Forecasts for the South Island show the wet weather should hit late tomorrow.

"It's looking pretty good for most areas on Monday - mostly sunny and cold starts," said Mr Duncan.

"Invercargill, Otago and even Christchurch should expect the showers later in the day on Tuesday as the clouds start rolling in."

He said the low coming in from the Tasman was unlikely to lift the icy temperatures in the South.

"Nor'easters tend to be quite cold, especially for the places like Christchurch.

"They're still looking at highs only around the 9 to 10 degree mark."

Mr Duncan also warned North Islanders not to get too comfortable with the warmer winter temperatures this week.

"When we get warmer weather like this, it's usually a sign of something worse to come.

"We're still of the belief we're in for another cold snap ... in the second week of July," he said.