Duncan said there would be a few showers in Auckland later this afternoon.
Clouds were already starting to roll in, with the cloud cover set to increase before the wind and rain set in.
"It will come in tonight and it could be quite heavy. It's got subtropical elements to it - the air that's being pulled down over northern New Zealand is subtropical,'' Duncan said.
"It tends to make the rain clouds more explosive and they produce these torrential downpours, which we've seen off and on for the last three or four weeks.''
Duncan said the areas most exposed to heavy rain in the very early hours of Sunday would be Auckland, Hamilton, Rotorua, Tauranga and Taupo, with Whangarei a possibility.
"Then it clears off to the east and moves into the South Island, and they get it.''
The rain would hammer the North Island for about 12 hours, with very heavy rain for about three to six hours.
The east coast of the South Island would then see heavy rain for up to three days.
Duncan said the low was the same weather system that had brought snow to Sydney.
"They've had inland snow from the southerly which is on the other side of this big low, and we've got the warm side for now.''
Strong winds had ripped up trees and roofs in Sydney, but Duncan said the wind was not expected to be as strong here.
The rain had already started in Northland, with northern and eastern areas of the country set to be most exposed later today.
Ranges in Northland could expect about 50-70mm of rainfall while parts of Coromandel and Bay of Plenty could expect 70-90mm.
Duncan said Waikato, Coromandel, western Bay of Plenty, Gisborne and Canterbury did not need more rain - but those were the exact same regions to be hit by the incoming low.
WeatherWatch said there was a high risk of flooding and slips in Canterbury and a moderate to high risk for more localised slips and flooding across Waikato, Coromandel, Bay of Plenty and Gisborne.
