"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 spokesman said.
There will, however, be a reprieve tomorrow morning and afternoon when rain is expected to ease for some time.
Aucklanders can expect rain settling in late this morning, with some heavy falls north of the city and strong winds later this evening.
A high of 16C and an overnight low of 12C is forecast for the City of Sails today.
As well as the heavy rain warning for the Coromandel Peninsula, the weather authority has also issued strong wind warnings for Auckland, Great Barrier Island and Coromandel Peninsula.
The warning is valid from 6pm to early tomorrow morning, about 8am, and will see winds turning to severe gales gusting up to 120km/h in exposed places during that time.
People are being told to stay alert, as a result, and to be extra careful when out and about.
"Strong wind gusts could damage trees, powerlines and unsecured structures," a MetService spokesman said.
"Driving may be hazardous - especially for high-sided vehicles and motorcycles."
Those in the Waikato, north of Tīrau, and Bay of Plenty, west of Matatā, are also being told to stay alert, as a strong wind warning is also in place from 10pm to 10am tomorrow. Wind gusts of up to 120km/h are also forecast in exposed parts then, also.
The gloomy weather is thanks to an intensifying low-pressure system settling over the North Island.
Weather experts said earlier that because the weather system was from the subtropics, it contains more moisture and potential for heavy rain than the average low-pressure system.
Despite a bad start to the first week back at school - on the weather front - the low is forecast to move away from the country on Wednesday.
