Heavy rain is falling in the region with warnings in place overnight.
MetService says people in Bay of Plenty, Rotorua and Coromandel can expect the heaviest rain from about 6pm tonight through to 11am on Wednesday.
"Heavy rain may cause streams and rivers to rise rapidly.
"Surface flooding and slips are also possible and driving conditions may be hazardous."
Locals are told they can expect between 130mm to 180mm of rain inland and between 70mm to 100mm of rain about the coast. Peak intensities of between 15mm to 25mm/hr are forecast on Wednesday morning.
Police have issued a warning for motorists.
"Poor weather reduces visibility and increases stopping distances, so remember to watch your following distances when in traffic and drive at a speed appropriate for the conditions," a spokeswoman said.
"It's also a timely reminder to do the basics to keep yourself and other motorists safe - make sure everyone is wearing their seatbelts, put the phone away, and don't drive while impaired."
MetService Meteorologist Amy Rossiter said warm days and humid nights are set to continue as a northerly flow pulls warm, moist sub-tropical air down over the country.
"These humid air masses are quite dynamic and can hold a lot of moisture,therefore producing significant rainfall rates. We are already seeing pulses of heavy rain affecting the upper North Island," Rossiter said.
Other regions with a heavy rain watch now are the north and east of Northland, Eastern Waikato and Taupō, Gisborne north of Tokomaru Bay and from north Taranaki (except the mountain) to Waitomo and across to inland Whanganui and Taihape.
The heavy rain and increasingly humid conditions over the last few days come as a newly named Tropical Cyclone Ruby has been located over the Coral Sea.
Its latest position is about 650km northwest of New Caledonia, as of this morning, and is moving southeast.
WeatherWatch head weather analyst, Philip Duncan, says it is currently a category 2 strong cyclone, but has the potential to become more fierce.
It is set to head over New Caledonia and some southern parts of Vanuatu tonight, before tracking towards New Zealand, Duncan said.
MetService says it is expected to intensify to a category 3 event as it moves across New Caledonia tomorrow.
By Saturday, however, weather experts expect that the cyclone's tropical characteristics will wane as it comes into contact with cooler seas.
"On Thursday, the system is expected to move southeast past the upper North Island as a deep low.
"Most outlooks suggest the system will remain offshore to the north of the North Island, but there remains some uncertainty regarding how close to New Zealand the system will come."
Next week, an atmospheric river of moisture may flow southward into Aotearoa New Zealand, as a tropical cyclone swirls to the north
There's a strong chance for a wetter than normal week across the North Island, with a risk for heavy rainfall that may lead to floodingpic.twitter.com/pfcyakXULt
Concern grows for Hayden Kemp, 40, last seen in Rerewhakaaitu on Friday.