Police are investigating a suspicious fire overnight which caused substantial damage to a vacant property in Brownfield Lane, Riverside.

At approximately 05:21hrs on the 4th February 2016 police were advised by Tasmania Fire Service (TFS) of a structure fire at 31 Brownfield Lane, Riverside.

Upon police arrival at the residence TFS were already in attendance and were in the process of extinguishing the fire.

Initial investigations by TFS indicate the fire started outside on the porch area at the rear of the residence.

The fire has spread through the middle section of the building and also the roof space of the shed causing substantial damage.

It is estimated the total cost of repairing the damage is approximately $120,000.

Anyone who saw any people or vehicles in the area of Brownfield Lane, Riverside, during the early hours of this morning should call Launceston CIB on 67773869 or Crime Stoppers on 1800 333 000.