Two people have died following a serious crash in Mangatainoka, in the Tararua District.
Police were alerted to the two-car crash at the intersection of State Highway 2 and Mara St around 5.45pm.
St John said three ambulances and two helicopters had been sent to the scene.
Three patients were being treated earlier - one in a critical condition, one with serious injuries, and one with moderate injuries.
NZ Transport Agency said a detour is in place for northbound traffic along Carisbrook Rd, Middle Rd and Kohinui Rd.
Those heading southbound should use the reverse of this.
The Serious Crash Unit is investigating.
Tonight's deaths come after a tragic week of fatal crashes that have claimed 22 lives throughout the country since last Friday.
Early this morning one person was killed in a single-vehicle crash on State Highway 50, in Tikokino.
The car crashed into a bridge around 1.20am near Glencoe Station Rd and Creek Rd.
Yesterday, another person was killed in a single-vehicle crash on Bainesse Rd, in Rangiotu, near Palmerston North, about 8.10pm.
On Tuesday, one person died following a serious crash on the Te Puke Highway, near Te Puke, about 5.40pm.
On Monday, nine people were killed on New Zealand roads.
A man died following a crash near Auckland Airport early Monday morning, after his vehicle left the road near the intersection of Tom Pearce Dr and Puhinui Rd.
The man was taken to Middlemore Hospital in a critical condition but later passed away.
Five people died following a single vehicle crash on Tirohanga Rd, in Taupo District, just before 7.40am on Monday.
They were 44-year-old Peter Rangikataua, 15-year-old Michelle Morgan-Rangikataua, 14-year-old Aroha Morgan-Rangikataua, 12-year-old Kahukura Morgan-Rangikataua of Rotorua and 26-year-old Rangi Rangikataua of Atiamuri.
Two hours later, three people died following a crash at the intersection of Mitcham Rd and Hepburns Rd, in Ashburton.
The crash occurred just after 10am and involved two vehicles. A fourth person suffered critical injuries.
The deceased were 30-year-old Chante Alissa Harmer, 19-month-old Te Awanuiarangi Shayelous-Jay Matenga Tapiata-Harmer, and 8-month-old Wysdom Amara Francis Tony Jane Tapiata-Harmer.
A motorcyclist was also killed in a crash at the intersection of Teapot Valley Rd and Waimea West Rd, in Brightwater, Tasman, around 3.20pm on Sunday.
This followed four road deaths on Saturday.
Two males died in a single-vehicle crash in Hawera, on the corner of Glover Rd and Ketemarae Rd, at 8.25pm on Saturday evening.
They were Tadhg McColl and William Wallace, both 18-years-old from South Taranaki.
Two people also died after a crash on SH1, in Topuni, Kaipara about 8.50pm on Saturday night.
Another person was transported to Auckland City Hospital in a serious condition, and a child was transported to Starship hospital, also in a serious condition.
A 19-year-old male driver is facing two charges of driving with excess breath alcohol causing death. Further charges are possible and inquiries into the crash are ongoing.
On Friday, a male pedestrian in his late 80s died following a collision with a car on Broadway, in Miramar, about 8.10am.
Soon after, a man died following a crash on SH1, in Kinleith, South Waikato.
He was 37-year-old Dwayne Adam John Marker of Taupo.
A man also died in a crash between a car and a motorcycle about 2.15pm on Friday, at Wairakei Terraces in the Taupo district.
He was later named as 59-year-old Colin Stephenson of Katikati.