Police say they believe 700 sheep stolen from a farm at Logan, 219 kilometres north-west of Melbourne, have probably been taken interstate.
Investigators say about $139,000 worth of sheep have disappeared in what appears to be a sophisticated operation involving several trucks and sheepdogs.
Leading Senior Constable Dan O'Bree said the farmer noticed the theft when he brought sheep in for shearing at the start of the month.
"We have had reports that there was a couple of four-decker trucks in the area around December at stray hours," he said.
"We think that the sheep would have to be walked off the property, maybe into some bushland for a bit of cover, (possibly to) a yard set up or ... some old yards nearby.
"So a fairly well-cleaned operation."
Police said that 197 Merino ewes with purple ear tags and 493 white Suffolk Merino cross lambs with red ear tags were stolen from the farm on Gowar-Logan Road.
"The sheep were stolen sometime since October when they were counted and the farmer has got them in late January to have them shorn and realised that he's 700 sheep down out of a mob of 1800," Leading Senior Constable Dan O'Bree, said.
Investigators believe the offenders would have needed to use at least two four-decker trucks if the thefts took place on one occasion.
LSC O'Bree said the offenders would have also needed to utilise a significant amount of portable fencing.
