Two children were injured when the car they were travelling in collided with a logging truck on State Highway One in Whangārei.
The car went down a bank after the collision north of Loop Rd in Otaika around 10am on Monday.
A logging truck went off the wrong side of the road into a ditch in the area affected by road works.
A 4-year-old boy was originally taken to Whangārei Hospital in a serious condition but the Advocate was later told he was fortunate to have avoided receiving any major injuries.
St John paramedics transported a second child - in a moderate condition - to Whangārei Hospital.
A police spokeswoman said neither driver was injured in the crash.
Two ambulances and a rapid response unit were dispatched to the scene alongside the Whangārei Fire Brigade.
Diversions were in place for more than two hours while emergency services and a clean-up team respond to the crash.
Northbounded drivers were re-routed via SH15 and SH14 through Maungatapere.
Southbound traffic was able to continue along SH1 but experienced long delays.
A witness at the scene said there was around 2km of backed up traffic as motorists were being diverted.
The road re-opened to southbound traffic shortly before 1pm.