He said police were still working to identify the victims and notify their families.
Police had recovered all four bodies.
"At this time we have recovered all the deceased, the vehicle is being removed from the beach and now we're commencing our enquiry into what took place," he said.
Mr Fergus said it was too soon to say whether speed or alcohol were factors in the crash, or if the occupants of the 4WD had been wearing seatbelts.
He said it was unclear why the crash happened. Reports of the truck hitting a log had not been confirmed and police found no log at the scene.
An Auckland Westpac Rescue Helicopter spokesman said the crew was tasked to an area of Muriwai, about 20km up the beach.
It appeared a vehicle had been travelling at high speed along the sand, close to the water's edge, where it has hit something - possibly hard sand - and rolled multiple times.
"On arrival, the vehicle was in the surf and three patients were deceased, having been ejected from the vehicle. A fourth person, trapped in the vehicle, was found alive."
Intensive care flight paramedic Russell Clarke said: "Our crew worked extremely hard to save him, but unfortunately our attempts were unsuccessful due to the extent of his injuries."
The crew transported the deceased male patients back to the Surf Lifesaving Club and left the scene just after 6pm.
Police initially said three people had died and a 12-year-old boy was critically injured. However, they later said all four occupants had died.
"Police and other emergency services are racing against the tide and fading light to recover bodies and evidence," a police statement said.
The police serious crash unit was investigating.
Footage on One News shows the 4WD badly damaged, standing in the surf, the tide swirling around and rising.
A police officer is seen alongside a digger and rope as the recovery effort began.
TV3 footage shows emergency services vechiles on the beach and what appears to be a body covered by white sheet.
Gloved police and firefighters walk along the tide line retrieving pieces of wreckage from the sand.
Other firefighters are seen dragging a tarp up the beach with a heavy object on it, possibly another body.
The crash happened about 3.45pm some 1-2km north of the southern end of the beach.
Muriwai Surf Club callout squad coordinator Mark Goodhew said he had taken the police to the scene of the accident, but was unable to elaborate further.
"There's a large amount of people doing it, small amount of trouble."
Mr Shearing said rangers usually managed the beach well and made sure drivers didn't get into trouble.
Muriwai resident Richard Pamatatau said he could see flashing lights north of the main settlement, but was unsure of their exact location.
He said most four wheel driving was done at the unpatrolled part of the beach, north of the riding centre and creek - the direction in which he could see the lights.
"These people driving on the beach are often travelling at great speeds so while an accident like this is tragic, it's not surprising."
Staff at Muriwai Top Shop heard multiple sirens as police and emergency services rushed to the beach.
Shop staff said it had been a busy weekend in Muriwai.
New Zealand Four Wheel Drive Association president Tony Burgess said there were a lot of people driving on Muriwai beach - including parents with kids - with little experience, and speed was possibly a factor in today's accident.
"The normal road rules apply when you're driving on the beach, and it's not as easy as it looks.
"You shouldn't twist and turn or break hard and you shouldn't go into the water," he said.
Mr Burgess said the organisation wanted people to be aware that there was a bit to learn before driving on the beach - and there were hazards, like logs, buried cars, and other beach users - to think about.
"We just want people to stick to the road rules really. And keep their seat belts on."