Auckland will have a high of 15C today with morning fog and southwesterly breezes.
The sunniest places to be across the country will be Napier and Gisborne - both fine and 15C.
Tauranga and Hamilton are both forecast a high of 14C, with possible drizzle.
Whangarei and Palmerston North have highs of 15C forecast.
Wellington will have a high of 12C today, with scattered showers developing in the morning.
By tomorrow, much of the rain will have passed off Auckland, with a high of 15C and some possible morning showers.
In contrast, a heavy rain watch covering inland Canterbury and Dunedin began at 9pm last night and is set to stay until 11am today.
North Otago may be even wetter with an overnight heavy rain warning set to stay until 9am today.
"Heavy rain may cause streams and rivers to rise rapidly. Surface flooding and slips are also possible and driving conditions may be hazardous," a MetService statement warned.
By midday, up to 70mm of rain is forecast to have fallen between Dunedin and Christchurch.
NZ Transport Agency has urged drivers to keep an eye on the changing weather conditions when on the highways tonight.
They have issued the following safe long-distance driving reminders: increase following distances and drive at a safe speed for the conditions, buckle seatbelts, plan your journey and prepare for delays, take regular breaks to fight fatigue and don't attempt to overtake unless completely safe.
NZTA stresses above all to not get behind the wheel if you've been drinking or tired.
For those journeying home after school holidays, regular traffic hot spots include: SH1 south from Northland to Auckland, the Auckland motorway system, routes from the Coromandel and Tauranga to Auckland, SH1 north of Hamilton, and SH1 coming into Wellington from the Kapiti Coast.
Motorists can also plan their journey and check up-to-date traffic conditions at journeys.nzta.govt.nz.