A Whangarei family's in shock after being assaulted by a man with a baton on a Northland beach.

Ivy, a woman who doesn't want to use her last name, said she was at Uretiti Beach on Friday with family and children, when a car sped by doing at least 80 kilometers an hour.

Ivy said her friend waved the car down when it turned around to return. She asked the driver to slow down as his driving could be dangerous with small children running around, Ivy said.

Then she said the driver smacked her fiance, Hadlee, in the face with a baton. 

"We were just trying to get him to go away. We had all of our kids there."

However, the driver accused of assault drove to Whangarei Police Station in his own car, while the family headed to Ruakaka Police Station to make their statements.

They said police told them their stories didn't match up and they might get charged with fighting if they pursued the matter further.