Significant rain has already fallen in the Gisborne ranges, ahead of a "deluge" for the eastern North Island on Monday.
MetService says 100-150mm of rain has already fallen in the Gisborne ranges in the last 12 hours, with 92mm in Whakatane.
Orange heavy rain warnings remain in place for the Bay of Plenty east of Matata and Gisborne/Tairawhiti north of Gisborne City from 9pm on Sunday until 9pm Monday.
"Expect 150 to 250 mm of rain north of Tolaga Bay and 60 to 120 mm farther south. These amounts are on top of what has already fallen. Peak rates of 15 to 25 mm/h north of Tolaga Bay during Monday morning," MetService warned.
And NIWA warns the start to the week is looking "particularly hazardous" about the northeastern North Island with a "deluge" incoming on Monday.
The cause is a moisture plume originating near Fiji, which could reach the thresholds needed to qualify as an atmospheric river, NIWA said.
"Rainfall forecasts suggest over a month's worth of rain is possible in the northeastern North Island."
With this, NIWA warns surface flooding and slips are likely, with a risk for river flooding about the far eastern Bay of Plenty, the Gisborne Ranges and East Cape.
Bay of Plenty Civil Defence warned residents to prepare for the wild weather on Sunday and Monday.
"We are well practiced with these weather events in the Bay, and as we know it's always important to be prepared," Civil Defence said on Facebook.
"Be prepared for any power outages by making sure you have torches and batteries available, and you've got some emergency food supplies in the house."
