Wednesday night will fall to a chilly autumn overnight low of 14C.
MetService said the week's weather would be influenced by a low pressure coming from the front lying off the Fiordland coast in the South Island.
This would slowly move north, bringing rain as well as cooler temperatures to the country.
"There is still decent rain forecast for regions in the North Island that are desperate for wet weather, so this is welcome news. However, this event alone is not set to be a drought breaker," MetService said.
Today's morning rain should hit from Whangarei through to Hamilton and Tauranga.
Whangarei can expect a top of 25C today with rain that will briefly become heavy.
Hamilton can expect a top of 24C with rain developing by late morning, before tomorrow heads for a high of 23C with the odd shower and a chilly overnight low of 11C.
Tauranga is set for 23C and rain, before climbing to 25C on Tuesday with the odd shower until evening.
Rotorua is likely to reach 22C with showers turning to rain in the evening before Tuesday drops to 21C and low of 10C overnight.
East coast residents in Napier can expect a high of 26C with light rain in the evening before morning showers and 26C tomorrow.
Morning rain will ease to drizzle in Wellington as it heads for a top of 19C with a top of 21C tommorrow and late afternoon showers.
The South Island's east coast meanwhile can expect sunshine and above average temperatures today.
That is expected to lead Christchurch to hit a high of 27C before rain arrives tomorrow morning and sends temperature plummeting back to 18C, MetService said.
Dunedin is looking at a top of 24C, before morning rain sends the thermometer dipping to 16C tomorrow and an overnight low of 9C.
Invercargill is set for 21C and a few afternoon showers and a top of 15C tomorrow along with an overnight low of 7C.
Nelson, meanwhile, is set for 22C today with rain easing to isolated showers.
Franz Josef on the West Coast can expect rain all day and a high of 18C.
