"It's going to be very unusually cold and snowy for October," according to NIWA.
About 20 slips have been reported over the weekend, the council says.
MetService said most of the country is expected to have single-digit temperatures by Tuesday night.
Footage yesterday shows at least a dozen vehicles were queued on the side of State Highway 29 in heavy rain.
Watches and warnings are in place for parts of Auckland, Northland, Coromandel and Bay of Plenty.
It's looking to be a very soggy start to the school holidays for some.
Northland, Coromandel and Bay of Plenty are also in the firing line.
The North Island and north and west of the South Island is expected to take the brunt of the warmer weather, including overnight.
One video shows a shark swimming just over the back fence of a property as the storm surge rises.
At least two people have died after the hurricane ravaged the island earlier this week.
The hurricane is tipped to reach Category 4 before hitting Florida this week.
Fiona came ashore before dawn Saturday (US time) as a post-tropical cyclone, battering regions with hurricane-strength winds, heavy rains and huge waves.
A severe thunderstorm watch is in effect until 9pm in Taumarunui, Taranaki, Taihape, Whanganui, Manawatu according to MetService.
MetService said the heavy rain could cause slips, surface flooding, sudden rises in streams and rivers, and hazardous driving conditions.
New Zealand just experienced the wettest July in recorded history, with massive storm systems dumping 150% more rainfall than normal across the motu.
MetService had warned of potential "severe thunderstorms" in the area.
Hawke's Bay, Tararua, and Wairarapa could be in for thunderstorms this afternoon.
NIWA Meteorologist Tristan Meyers told Seven Sharp while unsettled weather may lie ahead there will be long, fine, sunny spells for Aotearoa.
Thousands of people were evacuated after Typhoon Nanmadol made landfall in southern Japan on Sunday.
The Japan Meteorological Agency predicted as much as 50cm of rainfall by midday on Monday, warning of flooding and landslides.
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