Floodwaters are threatening to cut off Katikati, in the Bay of Plenty.
State Highway 2 has already been closed to the north and heavy rain is causing major flooding on roads south of the township.
Police said on its Facebook page the heavy rain had caused flooding in low-lying areas.
Delays are expected and alternative routes were being looked at.
Flooding has eased in the Western Bay of Plenty as the front that closed roads and threatened to cut off towns moves south.
Western Bay of Plenty District Council community relationships manager Frank Begley said the lunchtime high tide had passed without any major effects.
At least one lane of all roads were open as far as he knew and traffic was being managed.
"Contractors are following up a number of reported slips," he said.
"Things definitely have eased so we seem to be through the worst of it."
Resident Sue Larsen posted on Facebook there was major flooding and the highway was closed at the Uretara Bridge.
The New Zealand Transport Agency advised traffic heading from Tauranga to take SH29 over the Kaimai Ranges as flooding would close SH2 for some time.
A truck has rolled on the side of the road on State Highway 2 north of Katikati after flooding has cut off the road. Photo / Alan Gibson
In Coromandel, the seaside community of Tairua was cut off to the south, as SH25 was closed from the intersection with SH25A at Hikuai. No detours were available.
However, the state highway east of Kopu, near Thames, had reopened after a slip, with a stop-go sign in place.
Here's some flooding pics of Hikuai. Thanks Jared Stein, Hikuai School Principal.
— WeatherWatch.co.nz (@WeatherWatchNZ) July 7, 2016
Just after midnight, the MetService warned those east of Warkworth to expect torrential rain.
An earlier storm warning was extended as the lightning and thunder showed no sign of abating.
The Fire Service was kept busy with around a dozen callouts to help homeowners in Warkworth, Leigh and Matakana cope with flash flooding in properties and on roads.
And Richard Woods posted: "thunder in Auckland keeps going tonight".
Icy conditions took their toll on motorists in the South Island this morning with police attending more than two dozen ice-related crashes.
"We have attended over 25 crashes across the district in the past four hours due to ice and black ice on roads in South Otago, Gore and Invercargill," said Southern District Policing manager Inspector Tania Baron.
She urged all drivers to exercise caution and drive to the conditions.
It included being aware of areas with black ice and to watch following distances and speed.
