Parts of Northland and Auckland have received a month's worth of rain since Sunday, and more is expected to fall tonight.
And when it finally subsides, gale force winds that could threaten trees and power lines will sweep in.
The constant deluge, due to a slow moving low pressure system that will move into the Bay of Plenty region tomorrow, has left authorities dealing with surface flooding and warning residents in Northland to travel only if necessary.
Meanwhile in Canterbury, where flooding in Rangiora forced the evacuation of Bainlea House rest home early this morning, forecasters say that heavy rain will continue; the sun is unlikely to force its way out until Friday.
Council figures in Northland show up to 140mm has fallen at Puhipuhi, north of Whangarei, from noon on Sunday to 7.30am today - the same as the average monthly rainfall for June in a 43.5 hour period.
The latest MetService forecast is predicting a further 80mm to 120mm rain in the 12 hours to 8pm in areas north of Kaikohe.
"Power cuts are possible, as are road closures, surface flooding and slips," said Graeme MacDonald, spokesman for the Northland Civil Defence Emergency Management Group.
"Consider the risks, take the necessary precautions and don't travel unless you really need to. Given the predicted winds, boaties - if they haven't already done so - should also make sure their vessels are secure."
The next 12 hour period will be "pivotal".
To compound matters, severe east to northeast gales -- with gusts of up to 130km/h -- are also expected for Northland and Auckland when the rain eases off tonight.
Vector is warning of possible power outages in Whangapararoa, Warkworth, Orewa and Waiheke.
Mr MacDonald said flooding was currently limited to low-lying, flood-prone spots, but that could worsen if the rain fell in bursts of 25mm to 30mm an hour in places.
"This obviously won't happen everywhere, but if it does, the rivers and streams there (which are already well above normal levels) might struggle to cope with that sort of intensity."
Officials will be keeping a close eye on usual flooding hotspots including Kaeo, Moerewa's Turntable Hill area, and State Highway 1 at both Whakapara and Mata. They are also mindful of high tide on the east coast at 5pm.
In Auckland, a further 60mm to 100mm is expected to fall before tomorrow morning.
MetService Forecaster Leigh Matheson said that Northland and Auckland residents would not get any respite from the weather until tomorrow morning.
"All through that region there will be significant gales tonight. It could be hazardous for eastern areas of Auckland. Tomorrow morning, everything should be much quieter. The rains should have eased right off and the winds dropped right out."
She said in Canterbury, steady rain will continue to fall, but by tomorrow the worst will have passed.
"They will get 50mm to 100mm today, on top of what they've had, mostly around the hill country but some on the coast as well. There's no respite for areas currently having flooding, but it should start to ease off tomorrow.
"The sun may turn up on Friday. It's a pretty wet week, is the general theme."
Ms Matheson said the slow moving system will move to the east of the North Island.
"It's expected to pull away from Auckland and Northland, and push into Gisborne and the Hawke's Bay for the next two days. The western Bay of Plenty is likely to get significant amounts of rain tomorrow. I wouldn't be surprised if we see 200mm in a two day period."
