Three teenagers had their cars impounded and licences suspended after reaching speeds of nearly 180km/h while racing in Nelson.
Police said one car was clocked at just over 150km/h and the other 179km/h on Whakatu Drive, about 11.30pm on Friday.
Both vehicles were being driven by teenagers, aged 17 and 18, who had passengers onboard, Sergeant Stu Wright of the strategic traffic unit said.
"These drivers are both inexperienced. They showed reckless disregard for their own safety, the safety of their passengers and that of other innocent road users."
It was "horrifically dangerous driving", Mr Wright said.