Northland road policing manager Inspector Wayne Ewers said police were interested in talking to anyone who saw the dark blue Mitsubishi Triton on the beach.
"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," he said.
Police were also interested in hearing from anyone who may have witnessed the crash or who may have stopped to help injured passengers, Mr Ewers said.
"We're also keen to hear from anyone who may have seen a person walking from the scene of the crash."
Eight people, including the three dead, were travelling home from the beach when the ute left the road on a right hand bend and hit a large tree.
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.
Police have asked for anyone with information to contact Constable Reuben Cohen at Dargaville police on 09 439 3400 or Crimestoppers, anonymously, on 0800 555 111.
