Heavy rain expected in various parts of the country is already taking a toll on Kaikoura.
There have been rock falls onto State Highway 1 at various locations in the area, and police are urging all drivers on SH1 both north and south of Kaikoura to use caution.
Rain warnings are in place for Auckland and other parts of the country, but flood-vulnerable Christchurch is expected to escape the worst of the weather.
MetService meteorologist John Law said severe weather warnings for heavy rain have been issued for Northland, Auckland and the Coromandel Peninsula well as the top of the South Island, with parts of Marlborough and Nelson expected to get a soaking.
"The heaviest rain will fall away from Christchurch," Mr Law said.
Auckland would be feeling the brunt of a northeasterly weather system, which hasn't occurred in a long time, MetService Auckland forecaster Georgina Griffiths said.
"We're in for an extremely wet few days."
Auckland could expect between 100mm and 150mm to fall between today and Wednesday, with tomorrow set to be the wettest day of the week.
The Christchurch City Council has installed flood mitigation pump operations in the Flockton and Dudley Creek areas ahead of the weather system that is moving in today.
Mayor Lianne Dalziel said the pumps would reduce the flood risk to the residents in vulnerable parts of Christchurch.
"Even if the expected forecast doesn't arrive, this will be a good test of the Taskforce's flood mitigation deployment plans", she said.
At midday no rain had fallen in Christchurch, and 40 to 45mm were forecast to fall before Wednesday.
Surface flooding and slips could occur in Northland due to the heavy rain, where the rain has already been falling today, Civil Defence Northland said.
Northland CD Emergency Management Group spokesman Graeme MacDonald said tomorrow afternoon was expected to be the period of most concern.
He said Kaikohe had 100mm of rain fall by midday, and Civil Defence was collecting data for other areas in Northland to monitor the situation. The rain was forecast to fall steadily until tomorrow.
Mr MacDonald said possible problems could include rapid rises in rivers and streams, resulting in surface flooding and the possibility of slips.
Northlanders are used to heavy rain causing problems, he said, and should be taking necessary precautions today.