"Heavy rain may cause streams and rivers to rise rapidly. Surface flooding and slips are also possible and driving conditions may be hazardous," the weather authority said.
It will, however, be fairly warm, with a temperature high of 18C and an overnight low of 15C.
It seems the overnight temperatures have been very warm this week, with a 34-year-old record broken in Auckland overnight - the warmest minimum temperature recorded in the month of August since 1988.
The overnight minimum temperature at Auckland Airport recorded 15.5C - breaking the record 14.3C in 1988.
MetService meteorologist Stephen Glassey said it was thanks to the warm air being brought over from the tropics.
Up to 20mm of rain fell around the Auckland region.
"It's still steady rain and it's windy as well. We're still forecasting for it to be heavier rain today in Auckland," Glassey said.
Up to 91mm of rain fell in Kerikeri in the 12 hours between 6pm yesterday and 6am today. Kaitaia also got about 68mm of rain throughout the same period.
People in Whāngārei have not yet had heavy rain - but it is expected today and tomorrow.
Strong wind watch
A strong wind watch is also in place over Auckland and Great Barrier Island with weather experts warning that northeast winds may approach severe gale in exposed places at times.
The wind watch is currently in place until 11pm tonight.
The same heavy rain warning is in place over Northland until 9am tomorrow.
Locals are told to prepare for between 150mm to 220mm of rain to accumulate - especially in the north and east. Peak rates of 15mm to 25mm/h is forecast today.
Bus, train and ferry services affected
The weather has also caused a few interruptions on the morning commute in and around Auckland.
Auckland Transport has issued a number of alerts early this morning; warning train commuters that a fence is blocking a rail line between Kingsland and the Morningside Train Station.
"Trains travelling to Swanson will arrive and depart from Platform 1 at Kingsland and Morningside. Expect delays to Western Line train services as the fence is cleared away."
On the water, the West Harbour to Auckland ferry service for 6.50am was cancelled due to weather conditions.
A downed power pole will see bus route 324 taking a detour and will miss several stops.
People who usually catch that bus are told to use stop 7897 on Avenue Road, in Ōtāhuhu, as an alternative.
Commuters are advised to use the Journey Planner feature on the Auckland Transport mobile app for more transport options.
