Sergeant Haydn Korach, of Mid North police, said the offending driver had pulled on to the right-hand side of the road and parked in the middle of the lane when the other vehicle came around a blind corner.
He had been on his way home from a work function and reportedly stopped to relieve himself.
Police are awaiting the result of a blood-alcohol test before laying charges.
The following evening, about 6.50pm on Friday, emergency services were called to another two-car smash, this time on SH1 near Rangiahua bridge.
In that case three people were taken to Bay of Islands Hospital by road while a fourth, who had to be cut from the wreckage by the Okaihau Fire Brigade, was flown to Whangarei by the Northland Rescue Helicopter.