The Coromandel Peninsula and parts of Bay of Plenty are in for a wet start to the second week of the July school holidays, with a heavy rain warning issued.

MetService says all of the Coromandel Peninsula, and much of the Bay of Plenty, including Rotorua, could expect up to 110mm or 120mm of rain in the 12 hours from 8am Monday into Monday night or early Tuesday morning.

They advise drivers to drive cautiously, for people to avoid low-lying areas, and to prepare for the rain by clearing drains and gutters so rainwater would not build up. Streams and rivers could rise rapidly, and the rain could build up as surface flooding and cause slips.

Forecasters have also issued a heavy rain watch for Northland, Auckland north of Whangaparāoa and the Tasman District around Motueka.

Those areas could be in for a period of heavy rain approaching the intensity of the orange warning criteria.

Thunderstorms were also likely for Northland and Auckland thunderstorms were likely, and gale strength winds in Northland.

A frontal system over the north Tasman Sea is expected to move over northern New Zealand today.

On Tuesday, July 16, a front preceded by a moist northeasterly flow is forecast to move southeast over the North Island.

"There is a moderate confidence that warning amounts of rain will affect the eastern Bay of Plenty and the far north of Gisborne," says a MetService spokesperson.

"The heaviest rain is expected in the morning. Rain is expected to ease Tuesday afternoon."

Showers are expected to affect many areas on Wednesday, but severe weather is considered unlikely.

On Thursday, July 18, a weak front may bring rain to some parts of the country. Severe weather is however considered unlikely. 

An area of low pressure and associated fronts are due to approach the North Island from the northwest on Friday.

"This system may bring severe weather, heavy rain and gales, to parts of the North Island on Saturday.

"There is a low confidence that warning amounts of rain will affect Northland already on Friday."