Forty-five CFA firefighters from 12 brigades fought the blaze, between Toolamba-Rushworth Rd and Pogue Rd, which started about 3.15pm.
Shepparton police Detective Senior Constable Leon Schroeter said the blaze looked to have been started from three separate fires lit on the side of the road.
They believe the fires joined together to form the large blaze, with 15 fire vehicles and three firebombing helicopters needed to get it under control, Det Sen Constable Schroeter said.
Firefighters managed to save a nearby house and yards containing horses and foals.
Ebony Noble, who reported the fire to the CFA, said a second blaze started about 20m down the road shortly after.
“All of a sudden, within five minutes, it was through the paddocks,” she said.
Det Sen Constable Schroeter said police were appealing for anyone with dash-cam footage or information to contact Shepparton Police Station on 5820 5777.
Alternatively, witnesses could fill out an anonymous report online at www.crimestoppersvic.com.au or call 1800 333 000.