Silver Fern Farms says store lambs are being sold at over-inflated prices and farmers are unlikely to make a profit on them later in the season.
Chief executive Keith Cooper says high prices that lamb is fetching offshore are flowing through to the farm-gate.
"It's driving the store market to heady heights," he said.
But, he says that makes it unprofitable for those wanting to buy store lambs now to fatten for selling later in the season.
The lamb kill is down about 14% on last year for the season to date.
Mr Cooper says mutton is the success story this season, with slaughter levels up 40% due to international demand for mutton.
Meanwhile, Federated Farmers says lamb prices are inching closer to its target of $150 per head.
The target was set just over two years ago when prices had been $58 per head for several years.