Serious Crash Unit senior constable Cory Ubels said it appeared a "people mover type vehicle" had failed to negotiate a moderate right hand bend in the road.
The car - carrying two males and two females - had slid off the road, tumbling and landing on its roof.
One of the female occupants was dead when police arrived. Some of the passengers were treated for injuries at the scene, but an HBDHB spokeswoman said none were transported to Hawke's Bay Hospital.
Mr Ubels said the stretch where the accident happened was "quite a nice bit of road". As there was no evidence of any environment issues at the time of the crash, it was more likely to be a driver issue.
The crash closed the highway stretch until about 3.15am yesterday morning.
Just 15 minutes later crash scene police and other emergency services were called to a second serious crash near Raupunga on SH2 north.
A police spokeswoman said one woman died in the crash - which happened 1km out of Raupunga toward Wairoa.
A second person escaped without serious injury.
Although Wairoa mayor Craig Little had not heard if the occupants were locals, he said a fatal crash was a sad thing to happen to any family.
"It is a road you've got to take care on," he said.
Both incidents are being investigated by the Police Serious Crash Unit. Last night a police spokeswoman said it was unknown whether either of the women was driving when the crashes occurred.
Although it was too early to comment on the causes of the crashes, Ms Kura said it was known seat belt wearing rates were a key feature in recent serious and fatal crashes across the country.
"As far as police is concerned, any death or injury is one too many, and everyone - including road users - need to do their part to reduce deaths and injuries on our
roads and in our communities right across NZ," she said.
"Police cannot do it alone. Road safety is everyone's responsibility."
Mr Ubels said he had seen similar instances - where there were several fatalities during a short period- "multiple times".
These latest deaths took this year's road toll in the Hawke's Bay area, from Wairoa District in the north to Tararua District in the south, to 14 - one more than at the same stage last year.
Yesterday police also released the name of the motorcyclist who died north of Eketahuna at the weekend as 58-year-old Glen Evan Morgan of Masterton.
Of the 14 who have died on Hawke's Bay's roads, at least eight of those killed were drivers of vehicles involved in the tragedies, and two were motorcyclists.
Nationwide, it has been a bad start to the month with eight people losing their lives on the road - as of last night.