A house was evacuated and a ride-on lawn mower burnt out when people shot fireworks from their cars in Selwyn on Saturday night.
Two cars were seen chasing each other on Newtons Road and shooting fireworks from their windows at about 7pm on Saturday, police said.
Senior Constable Chris Caldwell said one firework caused a fire that spread across several hedgerows, gardens and set fire to a ride-on-lawn mower, before threatening a house, which had to be evacuated while the fire was put out.
Mr Caldwell said the people in the cars may not have realised they started a fire.
An eyewitness said one car was a silver BMW and the other a blue Mazda with white writing on the rear windscreen.
Police are appealing for information from the public.