Police were urging all motorists to exercise care and be aware of surface flooding.
The crash, which happened between Main and Galaxy Rds, has blocked the road for several hours and motorists are asked to follow the directions of police and other emergency services staff.
A St John spokeswoman earlier said St John was notified of the crash about 1.05 pm and three people were initially reported as being in a critical condition but one person's condition was downgraded to serious.
She said two people were taken to Waikato Hospital by road ambulance and a third person was flown to Waikato Hospital in a critical condition.
St John said five ambulances attended the crash scene as well as four helicopters, two from Hamilton, and one each from Tauranga and Taupō, she said.
UPDATE 4:30PM Due to a serious crash, SH1 in Kinleith remains CLOSED between SH30 and SH32. Avoid the area or be prepared for a detour as the road is likely to remain CLOSED for sometime, as a serious crash investigation takes place.^MF https://t.co/odF5qXQBgo
— Waka Kotahi NZTA Waikato/BoP (@WakaKotahiWaiBP) July 21, 2020
Four managers and a rapid response unit also attended the crash, St John also said.
A Greenlea Rescue Helicopter spokeswoman confirmed four rescue helicopters were sent to the crash.
She said the helicopters were dispatched at 2 pm and one of the Waikato helicopters took a 30-year-old male to Waikato Hospital in a critical condition.
All of the helicopters have now left the scene.
Fire and Emergency New Zealand confirmed they had also been called to the scene and had sent three fire appliances.
According to the New Zealand Transport Agency State Highway 1 remained closed at 4.30 pm and a serious crash investigation was underway.
