New Zealand's weather bureau said a low-pressure system off the east coast could bring further bursts of heavy rain through Tuesday, warning that rivers and streams could rise rapidly and that landslips were possible.
Large waves and dangerous sea conditions are also expected, MetService New Zealand said in its latest update.
A local state of emergency was declared on the Banks Peninsula near Christchurch, after flooding, fallen trees and landslides disrupted communities, and cut communication and power in some areas.
"We anticipated the weather easing off, but unfortunately that hasn't happened, and isn't forecast to begin easing until 6pm (4pm AEDT)," Christchurch Mayor Phil Mauger said.
Mauger urged residents to conserve water as the wild weather continued, while some households were told to boil water for drinking after flooding damaged a water treatment facility.
The tourist town of Akaroa, about 90km, northwest of Christchurch, was cut off.
Local cafe owner Cameron Gordon said the water had reached the walls of his business.
"Worst I've seen in my 20 years by quite some margin," he told NZME media group.
Online images showed collapsed sections of road, flooded streets and fast-rising streams across the region.
The storm earlier caused widespread disruption across large parts of the country's North Island, where flights were cancelled, major highways closed and power cut to tens of thousands of residents.
Several people in Wellington on the North Island remained without electricity on Tuesday, New Zealand media reported.
