"It's a van containing nine people," police said in a media conference this afternoon.
"Tragically seven of those people are deceased.
"It's just absolutely tragic how in the blink of an eye seven lives are lost."
"At the moment were still establishing what happened, but it appears the van has crossed the centre line."
Police understand a child who was killed was an infant.
Picton Volunteer Fire Brigade chief fire officer Wayne Wytenburg said he was "lost for words" about the crash.
Wytenburg said on Facebook "today we attended a most horrific vehicle accident with other emergency services. I'm lost for words to describe what our emergency services had to deal with. Our thoughts go out to the families."
He posted an update thanking locals for their supportive comments.
"The team is pretty resilient and coming to terms with what has [happened]."
Police said the head-on crash north of Koromiko involved a van and a refrigerated goods truck. It happened about 7.30am on Sunday.
"Along with the deceased, one person has critical injuries and one has serious injuries. The injured were flown to Wellington Hospital," said Inspector Paul Borrell, Tasman District Commander.
The driver of the truck suffered minor injuries.
The highway south of Picton remains closed while emergency services attend the scene, and is likely to remain closed for most of the day.
Also at the scene are staff from the Marlborough Disaster Victim Identification (DVI) and the Serious Crash Unit.
"Any death on our roads is a tragedy, let alone multiple fatalities in one crash," Borrell said.
"A crash of this scale has a huge impact on both the community where it happens, that of the deceased's family and community, and on emergency services who attend."
Borrell said police's focus remained on ensuring victims are supported and that the circumstances of the crash are fully investigated.
A St John spokesperson said three ambulances, two helicopters and a first responder unit were sent just after 7.30am.
"We assessed and treated three patients; one with critical injuries and one with serious injuries both flown to Wellington hospital and one with minor injuries transported to Wairau Hospital via road."
A police spokesperson says the road will remain closed for a significant period of time while emergency services, including the Serious Crash Unit, attend the scene.
Koromiko resident Trish Rawlings said she heard two big bangs around 7.45am, followed by sirens.
She went out to see a Big Chill truck lying on its side on State Highway 1, along with police, ambulances and fire trucks.
"A truck ... had gone through the railings. It was sitting between the railings and the railway line, facing Blenheim direction."
At that stage she was not aware of a second vehicle being involved, but after walking closer she could see what looked like a van with its boot open, obscured by ambulances.
The highway was not normally busy at this time on a Sunday, she said, and there was no intersection where someone could have pulled out.
The crash had happened on a "bit of a corner".
Rawlings said it was the same spot where local man Gary Kenny had died in a collision with a truck two years ago.
Michael Roberts, chief executive at Big Chill Distribution, confirmed one of the company's trucks was involved in the crash.
He said it was too early for the company to comment as the "event had really just happened in the last few hours" and full details are still not known.
The deaths bring this year's road toll to 184 - 27 more than the same date last year - and make this morning's crash one of the deadliest in New Zealand's history.
The most lives lost were in a bus crash in 1963 in the Brynderwyn Hills in Northland. Fifteen of the 36 people aboard the bus died after their bus brakes suddenly failed, sending the vehicle down a sheer cliff face.
In 2005, nine people died when their tourist bus collided with a logging truck near Morrinsville, while the truck driver escaped major injury.
Eight people including two babies under 2 died in 1995 when their house bus lost control on a bridge and plunged 50m into a Hawke's Bay river.
And in 2018, seven people died in a two-car collision in South Taranaki. Passenger Ani Nohinohi lost her partner and two daughters, while four elderly people in the other vehicle also died.
Road closures, ferry schedules change after crash
Waka Kotahi NZ Transport Agency said SH1 at the south of Lindens Rd intersection is expected to be closed all day.
Detours are in place on SH6 (Nelson to Blenheim), and for those travelling north via Queen Charlotte Drive from Havelock to Picton. The Queen Charlotte Drive detour is suitable for light vehicles only. Motorists should allow for 60 to 90 minutes of additional travel time while the detours remain in place.
Waka Kotahi staff attended the scene of the smash and are assisting the serious crash unit with its investigation.
"This crash will have a devastating impact on the families and throughout the wider communities of all those who have died. Our deepest sympathies are with them at this incredibly difficult time," Waka Kotahi Director of Land Transport Kane Patena said.
RNZ reports that KiwiRail will adjust ferry sailings in response to the road accident.
Interislander executive general manager Walter Rushbrook told RNZ the Aratere and Kaitaki sailings from Picton today would allow extra time for customers to reach the ferry terminal through the longer alternate route.
"We are working closely with our customers booked on ferries today, to let them know of the road closure.
"We are also working to reschedule any customers who will miss these sailings."
