Lamb prices indicate they are "running out of steam", ASB's senior rural economist Nathan Penny said.
In the latest ASB Commodities Weekly, Mr Penny said the last 18 months or so had been "stellar" for lamb prices.
Prices in 2019 opened at $7.20kg, some 45c/kg lower than where they ended 2018. From the peak price of $8.43kg in September, the per kg price had now fallen $1.23kg, or about 15%.
That fall was a little more than the bank had expected at this stage.
"The average fall over the past five years from the spring peak to autumn low is around $1.20kg.