"People could have easily died in any of these crashes," Mr Goodall said.
MetService has forecast showers today and tomorrow across the region.
Mr Goodall said it was frustrating to think all the positive results from the summer road patrolling could be undone by two days of wet weather.
"After one of these long, hot, dry spells, when the rain comes back the oil and debris tends to sit on the roads and has not been washed away by regular down-pours which makes the roads invariably slippery," he said.
He warned motorists need to slow down and take caution with following distances to allow more time to stop if necessary.
Mr Goodall said motorists responded positively to the 4km/hr tolerance in December and January and the crashes over two days jeopardised the road toll.
Northland's road death toll in 2013 was 21, up from a record low of seven in 2012.
The road toll for the region so far this year stands at one - on January 18 54-year-old Bill Bayliss was killed while cycling on Kapiro Rd, Kerikeri.
Police said that on Thursday, Waitangi Day, there were at least five crashes on Northland roads in the wet conditions.
11.15am - An elderly woman received minor injuries after her car left the road and ended up in a ditch on Pipiwai Road, Whangarei. She had to be cut from the car and was taken to hospital by ambulance.
11.45am - One person received minor injuries after a car crashed on State Highway 11, between Kawakawa and Paihia.
12.37pm - Three people received minor injuries following a two-car crash on Western Hills Drive. The cars were towed from the scene.
12.39pm - A two-car crash on State Highway 1 near Kauri, north of Whangarei resulted in two people being injured and taken to hospital by ambulance.
12.46pm - A car spun out of control near Opua and went off a bank. Signs are now being erected to warn motorists of the slippery conditions, as other vehicles have crashed in the same place during the past two days.
1.13pm - A nose-to-tail on Otaika Road, Whangarei, occurred but resulted in no injuries.