Police have named the three people who died in a crash on Whananaki North Rd, near Whangārei, early on Sunday morning.

Maurice Kareko, 35, Rhiannon Walters, 11, and Alec Walters, 5, died after a car crashed into a tree at around 2.15am.

Another child remains in Whangārei Hospital in a serious condition, while a fifth occupant of the vehicle received minor injuries.

The road was closed overnight while emergency services conducted a scene examination.

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Police haven't been able to confirm what caused the incident and investigations are ongoing.

The crash came after Northland police posted on their Facebook page on Saturday warning of a high crash risk.

"Take extra care on Northland roads today. After a long dry spell there may be oil, fuel and other contaminates on the road surfaces. These will become slippery when mixed with water," the post said.