Four people were injured in a head-on collision at Ngunguru, prompting police to issue a warning about slippery roads after periods of hot weather.
Police spokeswoman Sarah Kennett said it appeared a car heading to Whangarei on Ngunguru Rd slipped on a patch of diesel as it rounded a sharp corner, and into the path of a car containing a toddler and a couple in their 60s.
The collision, which happened at 9.15am yesterday, about 20km from Whangarei near Kaiatea Rd, saw all four occupants of the cars taken to hospital.
A 39-year-old man was taken to Whangarei Hospital in a serous condition with chest pains, Ms Kennett said.
A woman in her 60s sustained moderate injuries with a suspected broken hip, while the man had a bad cut on his head. The toddler, a boy, was shaken with minor injuries.
Ms Kennett said both cars had to be towed from the scene, with Whangarei District Council sending staff to clean up the oil slick.
The crash was a reminder for drivers to take care when roads were wet following dry period, as oil tended to "come to the surface".
"It's a reminder to be aware - when we get hot conditions then rain, people need to be very careful of diesel and oil on the road.
"People just drive as per normal - but particularly after it's rained the roads get slippery," Ms Kennett said.