However, police said nobody was located with the vehicle.
"Police would like to hear from anyone who saw any suspicious activity in the area," a spokeswoman said.
Ravensdown chief executive Greg Campbell told RNZ that a car crashed through a fence at the plant and into a building and caught fire.
"We don't know how or why that has occurred," he said.
Fire and Emergency New Zealand said the car was reported on fire on the ground floor of the building and two fire trucks and a supporting unit attended and found a small sulphur fire.
But the fire quickly escalated and spread through part of the five-storey building.
An additional seven fire trucks were dispatched as a result.
The blaze was eventually brought under control by firefighters and was declared out at 5.45am on Sunday - almost four hours after the initial call.
RNZ reported that the fire may have been ignited by the crashed car's hot exhaust, but fire investigators refused to confirm this on Sunday.
In December 2016, firefighters battled a sulphur fire at Ravensdown Napier Works, with 12 fire trucks and 60 firefighters attending the blaze.
At the time, there was about 800 tons of sulphur in the building.
Campbell told RNZ there was no structural damage and the plant would open for business as usual on Monday after the "unusual" incident.
A police spokeswoman said an investigation into the circumstances is under way.
Anyone with information is asked to contact police on 105, quoting file number 201213/3460. Alternatively, you can contact Crimestoppers anonymously on 0800 555 111.
