Waka Kotahi has encouraged motorists in Northland to either delay their travel if possible or be alert and brace for heavy rain and thunderstorms on their journey. 

Weather forecasters have issued a severe weather warning for Northland until midnight tonight. 

MetService meteorologist Kyle Lee said 106.8mm of rain had fallen in Whangārei in the past 24 hours, almost as much as the historical monthly average for the region for April (108mm). 

MetService also warned of a low risk of small tornadoes which could be strong enough to cause damage. 

Lee said on and off showers could be expected in Northland and Auckland until at least tomorrow morning. 

Another heavy rain watch had been issued advising that periods of rain and thunderstorms were possible for eastern parts of Auckland north of Orewa, Great Barrier Island, Coromandel Peninsula and Bay of Plenty west of Tauranga. 

"Showers with a moderate risk of thunderstorms continue into tomorrow for the upper North Island," MetService tweeted. 

For the Coromandel region, MetService said: "Heavy rain may cause streams and rivers to rise rapidly. Surface flooding and slips are also possible and driving conditions may be hazardous." 

The weather warning is in place until 1pm tomorrow. 

Waka Kotahi systems manager Northland, Kobus Du Toit, said the region's mild and sunny weather from the weekend had changed dramatically overnight. 

"If you are in Northland today and your plans are flexible, we suggest you delay travel. For those needing to head home from their Easter break, we urge you to remain alert while driving and expect the unexpected," Du Toit said. 

"If you must travel, drive to the conditions, slow down and maintain a greater following distance between your vehicle and the one in front. Stay alert for unreported hazards, such as surface flooding, slips, fallen trees and branches, and treat any downed powerlines as live." 

Waka Kotahi predicted traffic would be the busiest between 12-5pm and the heaviest traffic would be between Wellsford and Puhoi on State Highway 1. 

At 3.40pm, the commute time between Wellsford and Silverdale on SH1 was 57 minutes compared to 35 minutes when traffic is free-flowing. 

To drive from Wellsford to the city via SH1 was taking motorists 1 hour and 18 minutes at 3.40pm. The route took motorists 54 minutes when traffic is free-flowing. 

With a heavy rain warning in force until midnight for Northland, a further 70-100mm of rain was expected to fall, mostly to the north and east of the region. 

At the peak, rainfall rates could reach 25-40mm per hour and localised thundery downpours were possible. 

MetService has also issued a severe thunderstorm watch for Northland until 10am on Tuesday. 

The forecaster said thunderstorms were expected in Northland over this period and it was possible the thunderstorms could become severe and bring localised downpours of 25-40mm per hour. 

Rainfall of this intensity could cause flash or surface flooding and could lead to slips while driving conditions could also be hazardous, with surface flooding and poor visibility due to the heavy rain.