Severe weather warnings are in place for north-eastern areas of the North Island, with heavy rainfall moving south from Auckland likely to hit the eastern Bay of Plenty hardest.
MetService said the torrential rain currently bombarding Northland and moving through Auckland will continue into this afternoon and spread across Coromandel to Gisborne by evening.
Meteorologist Robb Kerr said that the area about East Cape could get up to 120 millimetres of rain, with localised downpours of as much as 40 millimetres in an hour.
The MetService said the Coromandel was likely to get 60-to-80 milimetres of rain this afternoon, and in the Bay of Plenty at about midnight.
Police said a heavy downpour of rain in the Auckland area led to a crash on State Highway 1 near Bombay this afternoon, with one car sliding off the road. They said the motorway had now been cleared.
The lines company, Vector, said a rain-saturated tree fell in Massey, bringing down a power line.
The Fire Service said it had had three callouts to minor floods caused by heavy rain, with a flood in Matamata cleared with a portable pump. Crews are still at the scene of floods in Mairangi Bay and Papakura in Auckland.
Forecasters are warning people to watch out for rapidly rising rivers and streams, and flooding on roads.
Meanwhile, The MetService has a severe weather watch in place for the lower South Island with a cold front to bring snow down to sea level in Southland and Otago, from tomorrow, and snow about the Milford Road.
The Transport Agency is urging caution on South Island roads, especially in the deep south, and farmers are advised to move stock to shelter.
Its Canterbury and West Coast journey advisor, Lee Wright, said motorists should check conditions before travelling.
He said anyone travelling into the alpine areas should carry chains and know how to use them, especially around Arthur's and Lewis Passes where snow showers are expected tomorrow night.