Earlier this evening, lines of thunderstorms were already producing high-intensity rainfalls across Northland.
Northland, Auckland north of Orewa, Bay of Plenty west of about Kawerau, and Coromandel Peninsula have red warnings in place until tomorrow. (Bay of Plenty's warning begins from 3am Wednesday).
Northland's rainfall forecast numbers are larger and the duration is longer than that for Auckland.
MetService meteorologist Geogina Griffiths is also warning of high intensity rainfall in areas north of Auckland city overnight. South of Orewa, localised flooding was possible, Griffiths said.
Meanwhile, Ngāti Wai Trust chief executive Huhana Lyndon says floodwaters have already cut off some areas around Whangārei. Lyndon says the rain is bucketing down, visibility is low and already some local roads are impassible.
She says a number of marae are open or on standby to open along the coast from the Bay of Islands to Whangārei.
Follow the latest weather updates here: