Mr Stevenson said highlanders grew reasonably quickly, "but there are other breeds in the UK that are equally as good".
The British National Farmers Union said the thefts were costing farmers a fortune and most of the sheep went into the foodchain.
Mike Tewson, the Devon farmer who lost 98 sheep, believed it would have taken at least two or three people and possibly a "good sheepdog" to steal his sheep.
"It would have to be someone with stock knowledge ... or a farm-related person," he told the BBC.
The Ministry for Primary Industries said live sheep were exported for breeding and none had been exported for slaughter since 2003.
Ministry figures, which did not record breeds, showed 361 sheep were exported this year, most to Australia but 71 were sent to Britain.
"Livestock are especially sought after because of their high genetic value," the ministry said, "and because New Zealand is free of most major exotic diseases."