Police say they are speaking to two young people about a fire at a Masterton landfill on Saturday.
Four fire engines and more than 16 firefighters were called to the blaze at the Nursery Road tip just before 7pm.
On Saturday evening, the fire service said the fire was under control, but they were still dampening down hotspots.
They said a single story building and a front-end loader have been burned, along with a large quantity of cardboard and about 2000 wheelie bins.
A nearby resident, William Ackerley, says he heard an explosion and saw thick plumes of black smoke rising into the air from the landfill.