The MetService has issued a heavy rain warning for parts of the country today and is warning residents it will likely to lead to rapidly rising rivers and streams.

The warning focuses on parts of the South Island along with the Tararua Range and Mt Taranaki.

The wet front is due to move onto Westland overnight before moving up the South Island Friday before slowly making its way up the North Island.

Between 140mm and 190mm of rain is expected to fall in the Tararua Range and Mt Taranaki from Friday to Saturday morning, and residents are warned the rain could cause rivers and streams to rapidly rise.

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Earlier today, MetService duty meteorologist Peter Little expected areas that were wet this morning to find the rain to eases this afternoon.

"Things are improving, there is an easing trend but we're stuck with the rain for at least this morning and then things will start to improve a little bit later on today."

Hamilton had been the wettest overnight, receiving 37mm since 10pm yesterday, which was a "reasonable amount" for the city.

He expects it to be quite drizzly this afternoon, with low cloud and even some fog settling in due to a humid air mass over the North Island.

However, there can't be complaints about the temperatures, as all of the North Island was sitting on double figures by 7am today.

"It's still going to be quite drizzly overnight and it's not looking that flash tomorrow. Saturday looks like there's going to be a lot more rain over much of the North Island and it's not until Sunday that we see more of an improvement.

"So we're in a few days of cloudy and generally wet weather, particularly most of today and also on Saturday."

However, areas from northern Waikato onward, can expect a mostly dry day tomorrow, particularly Auckland, he said.

"It looks like on Friday much of Taranaki is quite wet whereas there appears there will be a boundary from Waitomo northwards where it's not quite as wet. Auckland northwards does look like it will be dry for much of Friday and Waikato is on the borderline while further south it does look wetter."

The prognosis for the South Island is similar. Rain eases today but returns tomorrow before easing again on Saturday and Sunday when there will be showers.

As for next week, the country will experience a short, sharp cold snap on Monday as a southerly change sweeps through, before it switches back to northeasterly bringing warmer temperatures but also rain.