Alcohol, speed and an overloaded vehicle contributed to the severity of an accident that killed three Brethren members near Dargaville on Monday.

Susie Stewart, 48, her daughter Sadie, 16 and family friend Jamie Wearmouth, 18, died when the 4WD ute they were travelling in hit a tree on Bayly's Coast Rd in Northland about 6.45pm.

Mrs Stewart and the two teenagers were members of the Plymouth Brethren church, previously known as the Exclusive Brethren.

Three others were seriously injured in the crash.

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Constable Reuben Cohen told the Herald that without alcohol, speed and the vehicle being overloaded - carrying eight passengers - the result of the crash would have been "significantly different".

Fatal crash scene at Baylys Coast Road, Dargaville. Photo / Michael Cunningham
Fatal crash scene at Baylys Coast Road, Dargaville. Photo / Michael Cunningham

Mr Cohen said an appeal by police yesterday helped locate "several witnesses who saw the vehicle driving on Ripiro Beach (known as Bayly's Beach) before the crash".

In the appeal, Northland road policing manager Inspector Wayne Ewers asked for anyone who has seen the crash or stopped to help passengers to come forward.

"We're also keen to hear from anyone who may have seen a person walking from the scene of the crash," Mr Ewers said.

"We have information suggesting the ute may have been driving dangerously on Bayly's Beach prior to the crash, and passengers were seen riding in the back tray of the ute on the beach. Again, if anyone saw this we're keen to hear from them."

Mr Cohen said the manner of driving at the time of the accident was still being investigated.

He said police were not prepared to release details about the level of the driver's intoxication, or the likelihood of what charges, if any, would follow.

"These details will be released at the conclusion of the inquiry. We are far from completing the inquiry at this point," Mr Cohen said.

The male driver and rear-seat female passenger of the dark blue Mitsubishi Triton ute received minor injuries, while the two males on the tray were thrown from the vehicle at the point of impact with the tree, landing a short distance away.

Mrs Stewart was the front-seat passenger of the vehicle, Sadie was the rear left-seat passenger and Jamie was in the middle rear seat.