Motorists inside a vehicle that crashed off a key ridge road in Auckland's Waitakere Ranges set off a personal locator beacon to raise the alarm.

Four people were injured, one seriously, after the early morning smash.

A Rescue Coordination Centre official says without the distress beacon those inside the crashed vehicle could have had to wait for hours before someone spotted the wreckage.

A large section of Scenic Dr was cordoned off as police investigated how the car ran off the road around 2.45am.

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Police say the occupants inside the wrecked vehicle set off the locator beacon at 2.45am to get help.

The Rescue Coordination Centre called police to advise them a personal locator beacon had been activated in the Waitakere Ranges.

Cellphone reception in the area was patchy and it was the only way for those inside the vehicle to let others know about they were in trouble, said a police spokeswoman.

Fire and Emergency northern shift communications manager Carren Larking said the accident happened on the ridge-top road near the Arataki Visitor Centre.

Emergency services were called to the crash around 3.15am.

St John Ambulance said four people were taken to Auckland City and North Shore Hospitals for treatment.

One person was in a serious condition and three others had suffered moderate injuries.

Rescue Co-ordination Centre senior search and rescue officer Conrad Reynecke said the distress signal was first picked up at 2.30am by satellite.

It was registered to a Tauranga company but no one responded to a phone number linked to the beacon.

Reynecke said police were asked to drive near the location. It meant those inside the wreckage were spared hours waiting for someone to drive past on a road 5km from the nearest suburb.

"Without that distress beacon they would have had to wait for a vehicle to come across the accident and at 2.35 in the morning when we got the alert it could have been several hours for someone to come along especially on that location," he said.

Auckland Transport earlier warned people heading into the city from west coast communities to use an alternative route, with the crash closing the section between West Coast Rd and Shaw Rd.

The road was reopened to traffic just after 7am.

The Police Serious Crash Unit is investigating the circumstances of the crash.