Two police cars have been set on fire in the car park at Lyttelton's police station.
The Fire Service was called to the Sumner Road station at about 2am on Tuesday when a neighbour reported hearing an explosion and seeing flames.
Sergeant Gary Manch says one car is a write-off and the second suffered exterior heat damage.
Sergeant Manch says because the area is a crime scene police staff have to use a ladder to get into the building.
