"We have information suggesting the ute may have been driving dangerously on Baylys Beach prior to the crash, and passengers were seen riding in the back tray of the ute on the beach," Mr Ewers said.
Police were also interested in hearing from anyone who may have witnessed the crash or who may have stopped to help.
"We're also keen to hear from anyone who may have seen a person walking from the scene of the crash," he said.
Eight people, including the three dead, were travelling home from the beach when the ute left the road on a bend and hit a tree about 6.45pm.
Three were seriously injured, while the driver and a female rear-seat passenger received minor injuries. Two males on the tray were thrown from the vehicle when it hit the tree, landing a short distance away. Both were taken to Whangarei Hospital with non-life-threatening injuries.
The three deaths take Northland's provisional road toll for 2016 to 18, compared with 22 for the whole of 2015.
Police have asked for anyone with information to contact Constable Reuben Cohen on 09 439 3400 or Crimestoppers, anonymously, on 0800 555 111.
