A Whangārei mum is glad she had her baby boy and 10-year-old cousin correctly restrained after the car they were in spun out and slammed into a roadside barrier.

In Northland about a third of all the road fatalities last year were attributed to people not wearing seatbelts and a recent police blitz earlier across the region this year found a third of people stopped were not correctly restrained.

Terry Henry, 19, was driving north on Western Hills Dr when she felt a bump at the rear of the Holden commodore which caused her to lose control and hit the roadside barrier about 11.45am on Mother's Day.

Fortunately baby Haki was strapped into his child restraint in the rear of the car, while his 10 year-old cousin was wearing a seatbelt and was in the front passenger's seat.

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"It happened so fast ... my car went to the left and hit the curb. I was trying to get control but the car spun round," Henry said.

After completing a 180-degree turn, the car ended up facing the wrong way. A female motorist stopped and started waving down traffic and took baby Haki while Henry rushed to her young cousin who had facial injuries after the airbags deployed, filling the car with smoke.

"I was just so worried she was going to be really hurt. I felt so guilty."

Both panels on the passenger's side of the car where the two children were seated were extensively damaged.

All three were taken to Whangārei Hospital in a St John Ambulance and released about 7pm after being checked over and monitored for a few hours.

Her younger cousin had two black eyes but otherwise was alright.

Henry said she felt a "bump" but did not see any vehicle at the time as things unfolded so quickly.

"There was a witness who saw a car boost off straight after we crashed. If anyone did see something can they please let police know."

She thanked the woman who held her baby and an off-duty medic who had been driving and stopped to help until the ambulance crew arrived.

"Be careful on the road whanau. You may be doing all the right things but you still have to be cautious of the others on the road and make sure all your kids are buckled in," Henry said.

Henry and her partner had saved for the car for seven months and only had it 48 hours before the crash. Unfortunately it was not insured at the time.