© NASA Terra/MODISEx-cyclone Debbie over Tasman Sea affecting New Zealand on April 4, 2017.
In Auckland, the north island's biggest city, a cliff collapsed on to an apartment block and more than 1000 homes were without power. A state of emergency was declared in two other districts - Whanganui and Rangitikei - as river levels rose amid record rainfall.
No fatalities have been reported but police were investigating reports a person was missing on the Waikato River on Wednesday afternoon.
Floods forced the closure of several country roads and highways throughout the north island, including Auckland. Schools remained closed on Wednesday in Whanganui and Rangitikei, and in Clevedon and Brookby in rural Auckland. Some train and ferry services were replaced by buses due to landslides and flooding.
Authorities warned that a lull in the rains early on Wednesday would not last. A renewed downpour over the next 24 hours brings the risk of flash flooding in areas still recovering from a devastating earthquake in November.
"Our ground saturation levels are very high which means our water table is elevated, therefore isolated flooding is highly likely," Civil Defence Controller Garry Towler said in an emergency statement.
The country's weather bureau, the MetService, said there was a risk of severe thunderstorms or severe convection over the north of the country, bringing a continued risk of localised downpours of 25 to 45 millimetres per hour.
"Rainfall of this intensity can cause flash flooding, especially about low-lying areas such as streams, rivers or narrow valleys, and may also lead to slips. Driving conditions will also be hazardous with surface flooding and poor visibility in heavy rain.
"These severe thunderstorms or severe convection also have the potential to bring strong wind gusts exceeding 110kmh, as well as the possibility of a small tornado."
New Zealand's mountainous terrain makes its roads susceptible to landslides and many regions are still recovering from November's 7.8-magnitude quake. Two cities were on alert for mandatory evacuation later on Wednesday.
Cyclone Debbie, a category four storm, one short of the most powerful level five, pounded Australia's Queensland state last week, smashing tourist resorts, bringing down power lines and shutting down coal mines. Six people have died since it hit.
Reuters, Stuff.co.nz
Comment: In Australia, more than 60 locations in Queensland affected by Cyclone Debbie received the highest amounts of March rainfall ever recorded, in many cases breaking records that had stood for more than 50 years.
Bureau of Meteorology climatologist Dr Jeff Sabburg said the extraordinary rainfall figures could largely be attributed to the low pressure system left over from the cyclone. "What was exceptional in causing the records was the amount of moisture in the air associated with ex-Tropical Cyclone Debbie," he said.
Damage from cyclonic winds and flooding has affected thousands of properties across Queensland, with more than 750 homes deemed uninhabitable. Electricity has been restored to about 45,000 people blacked out by Cyclone Debbie but by yesterday morning almost 20,000 homes were still without power.