King tides and swells are breaking over the Bay of Plenty and may cause flooding in low-lying areas.
MetService says waves were 3.5 metres high on the east coast early this morning and causing sea levels to rise.
Between 70 to 100mm of rain is expected today, mainly near the Kaimais, with peak intensities of 20 to 30mm/hr, with localised downpours possible.
Metservice meteorologist April Clark says the "biggie" today would be the effect of king tides arriving at the same tide north-easterly winds whipped up heavy north-east swells.
"The swell and king tides mean we are looking at the potential for sea inundation on the eastern coast from Northland through to Bay of Plenty."
There was still some uncertainty because if winds were not as strong as expected the swell would be smaller, but she urged people living in coastal areas to be aware.
Coromandel Peninsula has had the most rain so far with 106.5 millimetres in the last 24 hours recorded up in the pinnacles.
Coromandel Peninsula should expect a further 60 to 90mm of rain, in addition to what has already fallen and peak rates of 20 to 30mm per hour, with localised downpours possible.
April says the weather is moving faster down the country than what was initially forecast.
"People may be seeing the rain a little bit earlier and it will be clearing a little bit earlier."