Meteorologist Angus Hines said there was a "little bit of uncertainty" about the amount of rain that would fall, but expected showers this morning which would turn into heavier rain tonight.
The "significant falls" were expected tomorrow.
However, he expected weather to be better for Easter holiday-makers.
"The general trend is for most of the severe weather to pass by Easter," Mr Hines said.
"The heavy rain should clear by the weekend, it's looking pretty good."
The Eastern Bay of Plenty would be the last place the high pressure systems would cross, with the heavier rain falling into Friday.
Rainfall accumulations could reach warning amounts in the Bay of Plenty, Northland, Auckland, Taranaki, Coromandel Peninsula, Rotorua, Taupo and the far north of Gisborne.
A band of heavy rain was expected to move onto Auckland and Taranaki tonight and then spread east during tomorrow.